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'Low Visibility' and how it affects your flight - an explainer

'Low Visibility' and how it affects your flight - an explainer

Old Jan 24, 23, 6:33 am
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Join Date: Dec 2012
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'Low Visibility' and how it affects your flight - an explainer


For many of us the weather reports on the news the day prior to having some flying in our working or leisure schedules can induce a slight sense of dread. Sometimes it is not always apparent why your flight is delayed or why it even should be in the first place. This guide is being written to hopefully answer some of the question and given far more detail as to why flights get delayed and to bust main stream media fed myth with facts. With that aside we need to answer the first question;

What is meant by fog and low visibility in Aviation?

Fog is the suspension of condensed water droplets in a parcel or body of air. For any given air mass there is an amount of water that can be held within it as a clear vapour. The amount of this vapour determines the relative humidity expressed as %RH. Generally speaking, the warmer a parcel of air is the greater its vapour carrying capacity is, and vice versa for cooler air. The vapour within an parcel of air will remain in such a state until it is cooled to what is known as the ‘dew point’ temperature. This temperature is dependant primarily on pressure, the higher the air pressure, the lower the dew point temperature will be. This is why the cloud base on a warm British summers’ day is often at a higher altitude than for a comparable day in the cooler winter months. As a parcel of air cools below the dew point temperature, the water can no longer be held as a vapor, and it condenses into tiny water droplets. This is the formation of a cloud, and the visibility within this cloud of water vapour drops will reduce depending on the density of these droplets. Essentially fog is just a cloud on the ground.

Fog by it’s aviation definition is a reduction in visibility below 1KM in EASA and UK and 1 Mile or 1600ft as per the FAA in the USA, caused by the condensation of water vapour.

Fog can form in a number of ways, these are briefly;

1. RADITION FOG - Forms due to overnight radiational cooling of the Earth’s surface, which in turn cools the adjacent air enough to cause condensation of the water vapour within the air. It is usually less than 300ft thick, localised and dissipates with strong wind or 2-3hrs after sunrise.

2. ADVECTION FOG - Formed when moist air moves over a colder surface. Thicker than radiation fog (>1000’) and more outspread. Wind strengthening may transform it into low stratus.

3. COASTAL FOG - Is a maritime advection fog with sea breeze (think San Francisco/Cape Town).

4. UPSLOPE FOG - Formed on windward slopes by the forced ascent of air causing adiabatic expansion and cooling. (Levante of the Rock of Gibraltar).

5. EVAPORATION FOG - Formed within a cold and stable air mass by rapid expansion from an underlying warm water surface.

6. FRONTAL FOG - Associated with frontal zones/ passages. Rain may fall into cold stable air and moistens it. Fog may also form when cold and warm air mix in the frontal zone or by the sudden cooling of air over a moist surface

There are of course other causes of reduced visibility. Primarily they are mist which is defined as a visibility of between 1 to 5KMs. Where as Fog exists in a parcel of air comprising 100%RH, Mist is generally with in the 70-100%RH bracket. Haze is a reduction of visibility due to tiny dust particles suspended uniformly in the air. Smoke can be formed in many ways any wild fires burning can carry these particles in a stable air mass (think high pressure dominating) with a steady slight breeze for many miles. Sand can also cause greatly reduced visibility. A sand storm can lift up particles sever thousand feet and reduce the visibility to mere inches.

Any reduction in visibility reduces our ability to navigate using our peripheral and direct vision and can lead to a loss of situational awareness. As anyone who drives a vehicle will know, when the fog rolls in, the traffic flow reduces.

But the airplane lands itself, right?

No, not normally. Take off and Landing are both visual manoeuvres and as we are about to discover in some greater depth than is normally made available, not all aircraft, crews, and even runways are the same. So whilst on some runways we can have the aircraft Autoland, it isn’t a guarantee and regardless, in any reduction of visibility, we do also need to take-off.

Some History

Implicit in the definition of Aerodrome Operating Minima (AOM) is the notion of an approach ban: if the conditions (usually RVR or Runway Visual Range) are worse than the prescribed minima, the approach must be discontinued. This philosophy arose out of work done following the crash of a BEA Vanguard at Heathrow in October 1965. Before that time the minimum RVR for a Cat I approach was 300 yds and the approach ban was not rigorously enforced. The crash lead to the setting-up of a government working party to consider the whole subject of AWOPS and its report is the seminal work on the subject. The UK also established the Blind Landing Experimental Unit at RAE Bedford whose work laid much of the foundation for the development of Cat III approaches. The application of approach bans was extended to foreign carriers after an accident involving an Afghan 727 at Gatwick in 1972, which crashed while trying to land in 100m RVR.

Some interesting results came out of the government-sponsored work, notably that there is no such thing as uniform, or homogeneous, fog. The conclusion from the report was that AOM should be established to provide safety, not to guarantee the success of an approach. If minima were to be set to ensure that every approach was successful cloudbase would need to be included and there would be no need for any kind of decision point. In fact, approach success is expected to be 99% for Cat III, down to 50% for Cat I and above. Actual go-around rates are very much lower than these numbers would suggest because most approaches do not occur in limiting conditions. Minima of 200 ft/550m for Cat I do not mean that an approach in those conditions will necessarily work, nor that one in less than 550m would definitely NOT work, merely that the balance of risk means that using less than 550m is a poor idea.

So how do Airports and Airlines keep going?

The answer to this is threefold as it depends on three separate things. Namely these are;

  • Airport Facilities
  • Crew Qualifications
  • Aircraft Capability

Broadly speaking working in low visibility conditions has two distinct elements. Low Visibility Procedures and Low Visibility Operations. Airports liaise with ATC to do the Low Visibility Procedures bit, and Pilots and Airlines do the Low Visibility Operations bit. The concept of Aerodrome Operating Minima, as approved by the regulatory body for the state the airfield is in, is the first basis in maintaining a safe operation.

Firstly, then the Airfield.
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Last edited by Sigwx; Jan 24, 23 at 6:52 am
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Old Jan 24, 23, 6:34 am
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Background –

Not all airfields are created equal. Some are on wide flat plains and others are stuck in a deep valley with mountains on both sides. Some are in densely packed cities and some are built on manmade islands in a body of water. Regardless of topography, all need to have an ‘Obstacle Clearance Zone’.

This is a zone centred over a runway that must be clear of obstacles apart from ‘frangible’ navigation aids such as ILS and Glide Slope antennae. This creates a protected path for the aircraft in both its landing and take off phases when close to the airport. An assessment is made of all obstacles in this area and the height of the tallest obstacle has an additional height added plus a ‘fudge factor’ to account for the aircraft type and airfield (big heavy vs small prop/narrow vs wide runway etc) This forms what is known as the OCH or Obstacle Clearance Height. It is from this OCH that airports and aviation authorities via set out procedure design criteria can create what is known as approach Minima or the stated visibility and altitude or height that an aircraft is allowed to operate in and down to before a landing is not allowed. As this guide is about low visibility we will focus on ILS or Instrument Landing System procedures.

An ILS provides precision guidance and a 3D approach (both lateral and vertical guidance are externally provided to guide the aircraft) down to either a Decision Altitude (DA) or a Decision Height (DH). DA is referenced to mean sea level and DH to the height above the landing runway threshold.

For a clear day or one where the cloud base is greater than 200ft off the runway and the visibility greater than 550m generally, we can fly down to a CAT 1 minima that would normally require 550m visibility and allow us to descend to an altitude 200 ft above the runway. We can do this based on a pressure altimeter.

The next few categories below a CAT 1 ILS approach place us into the realms of low visibility and we need a radio altimeter to go to a lower height.

Runway Requirements

In order to fly anything other than a simple approach in clear, calm, benign weather conditions we need to have sufficient lighting in order to be able to firstly see the runway when we arrive at the lowest permitted height, and secondly to know where abouts we are in relation to that runway.

A difference in the type of lighting found at a small quiet airfield vs a large international capable of supporting and executing low visibility operations is seen below

As you can see, for approach and landing CAT II and CAT III operations in low visibility we need a lot more lighting. We need to have;

RUNWAY EDGE LIGHTING – they mark the side edges of the runway, are white and need to be spaced no greater than 60m apart. 600m or a 1/3 of the runway distance from the end and these lights may be coloured yellow.

RUNWAY END LIGHTS – they are coloured red and aimed towards the landing direction and mark the end of the Landing Distance Available, there must be at least 6 of them spaced no more than 6m apart.

THRESHOLD LIGHTS – Spaced at 3m apart they are green and displayed at the very beginning of the threshold in the direction of landing.

TOUCHDOWN ZONE LIGHTS – these extend in two rows of barrettes into the runway beyond the Threshold lights and sat either side of the runway centre line. These are a specific requirement for CAT II and CAT III ops. They are formed of 3 lights spaced 1.5m apart max and each set are between 18 – 22m apart dependant on runway width. They will travel along the runway for 900m and the spacing for each pair needs to be 30 or 60m with 30m enabling a lower set of values to be used for height and visibility for landing aircraft.

CENTRE LINE LIGHTING – a typical feature of most runways, but for these operations it needs to be colour coded. At 900m from the end of the runway it alternates between red and white, and with 300M to go until the runway end lights, the centre line lights are all red.

RETILS – Rapid exit taxiway indictor lights a 3 sets of lights that are arranged 3 then a pair or 2 and then a single light that marks the start of the turn off lights (think motorway exit countdown markers). These taxi way lights that lead off the runway will be alternating yellow and green whilst we are in the ILS CRITCAL AREA.

RUNWAY VISUAL RANGE TRANSMISSOMETERS – These are devices that measure the visibility and are placed to the side of the runway shoulder at the touchdown, mid-point and stop end of the runway and are permanent fixtures.

Taxiway Requirements

The lighting and marker boards which will be illuminated are seen below.

Essentially the green centreline lights will be spaced no more than 30m apart and even as close at 7m at junctions and corners. There will be blue taxiway edge lighting and as we approach the ILS CRITCAL AREA there will be a switch to alternating yellow and green centreline lights.

The runway entrance will be guarded by signage, and for CAT II and III there will be a sting of what look like roman numeral 2s or IIs. This I further enhanced by yellow flashing ‘wig-wams’ by the taxiway sides and possibly inset flashing yellows either side of the taxi way centre line. In the USA there are also red lights to ‘block’ your path that are controlled automatically by ground radar sensing aircraft on or approaching runways.

ILS CRITCAL AREA – The operation down to CAT II can result in either a manual landing (with specific training) or an Autoland. ALL CAT III approaches must end up with an Autoland. In order for this to be safe the ILS signal which is guiding the aircraft must be protected. Metallic objects such as ground vehicles and aircraft can bend, and deflect this signal which can affect the following aircraft’s approach.

To prevent this nothing other than an aircraft landing must be anywhere near the runway and ILS CRITICAL AREA that extends either side of the runway shoulders. This is the significance of the yellow and green alternating centreline lighting. It marks the ILS Sensitive area. And is protected three fold. Firstly pilots will vacate safely and report runway vacated once the tail is clear of this area, this signals that we will no longer be affecting the following aircraft’s takeoff roll or Autoland.

The ground roads will have stop signs, marker boards and warning symbols to alert ground based vehicles and usually Airfield Ops of the airport will be ensuring CAT III safeguarding to ensure the critical and sensitive areas are sterile.

This CAT III safeguarding forms part of Low Visibility Procedures conducted and executed by the airport and ATC. It is promulgated in many ways buy the decision to enter the procedures is normally based on visibility alone and sometimes cloud base, airfield dependant. As an example for Heathrow the criteria are either the runway visual range is 1000M and expected to fall to 600M or the cloud base is at 300ft and expected to fall to 200ft.

For Airfield Ops it is a case of ensuing road signs are working and appropriate lighting functioning and to monitor for any possible infringements of critical and sensitive areas. For ATC the approach controllers will be needing to increase the spacing on the approach between aircraft to ensure a sufficient gap for the preceding aircraft to vacate and no longer be interfering with the ILS signals prior to the following aircraft getting to a critical height. All operators and ground staff will be made aware of the safeguarding procedures commencing and that the airfield is entering low visibility procedures. Once the safeguarding is completed, ATC is notified and only then may ‘Low Visibility Procedures’ commence.

This at capacity constrained airports is what leads to delays for inbound aircraft. For forecast low visibility conditions London ATC will proactively set via eurocontrol a ‘flowrate reduction’. This maybe down to half of the normal rate in which Heathrow, Gatwick, Stanstead et al land aircraft on a lovely day. Those aircraft already airborn will be told to slow down in order to reduce the amount of time they are holding in the holding patterns prior to commencing their approach. To prevent a log jam, these not already departed will have a delay placed on their flight. Dependant of forecast severity and duration, some flights may also be pre-emptively cancelled.

Fog is however a tricky mistress to play with and it can persist or dissipate out of forecast expectations. As soon as the flow rate can be increased ATC will want to open the tap as quickly as it can in order to prevent further dieback of log jammed aircraft. It is for this reason you find yourself boarding more or less on time and then being sat there waiting. ATC need to know that the second they say ‘’go’ that you are ready to react promptly.

The only other real pressing issue is the ILS itself which much be calibrated at regular intervals and have an immediate power back up supply capable of taking over instantly should the primary power source fail.
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Last edited by Sigwx; Jan 24, 23 at 7:07 am
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Old Jan 24, 23, 6:35 am
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In order for pilots to be able to conduct low visibility operation in conditions below those allowed for CAT I the following need to be satisfied for UK operations.

Ground training – This is conducted during the ground school period of a pilots conversion course onto an aircraft type.

Flight Simulator Training – May be substituted for the real aircraft if more relevant, however for the vast majority of airline pilots this will be simulator training conducted at the airline’s relevant training centre. For BA this is housed in TBA at the eastern end of Heathrow.

Operator’s specific check or line check – The LV or Low Visibility qualifying check is normally conducted as part of the license skills test or LST. The final simulator qualifying check prior to base training.

After qualifying on type and in possession of a new hot off the press type rating in their license, a pilot will conduct one AUTOLAND during line training in the real aircraft. Captains are precluded from conducting low visibility operations until they have achieved 20 sectors and 50 hours on the aircraft post release to the regular line environment.

The ground training consists of what you are reading in this guide, plus a little more depth that isn’t vert pertinent to understanding everything that goes into this type of adverse weather operation.

The simulator phase will consist of at least 6 approaches and include, normal operations, non-normal operations and low visibility take-offs, go arounds and rejected take offs too.

So as you can see there is a regulatory element to the qualification and we need to re-qualify every year in our instrument rating revalidation. Should a situation develop in which we do not have sufficient time to complete the low visibility elements of the check and re-validation, we are precluded from operating in those conditions until we can be scheduled back into the simulator to complete these elements.

Last edited by Sigwx; Jan 24, 23 at 7:09 am
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Old Jan 24, 23, 6:36 am
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In order to Autoland an aircraft needs to have an Autoland system, not all aircraft do, typically smaller turboprop aircraft and some older regional jets, and some operators of aircraft capable of it refuse to pay for the cost of the certification, training and maintenance programmes required by operators in order to have and to hold the certification to carry out such manoeuvres. This is typically due to the cost benefit in their operation where such conditions are one off events. Typically these operators will conduct training and obtain approvals to operate to CAT II limits in order to offer some protection to their operation. Again, this is operator to operator dependant, and some won’t bother.

For aircraft with Autoland systems installed, these are normally integrated within the suite of autopilot functions. The aircraft manufacturer will have flight tested these functions during the type design certification process with the relevant worldwide authorities in states with operators that the type is to be sold to.

Typically an Autoland system falls into two distinct groups;



A FAIL PASSIVE autopilot is one whereby a failure of the system or any ground based component (ILS signal) won’t leave the Aircraft misbehaving or give any out-of-trim handling condition to the Pilot. That back up is indeed the Pilot should the system suffer a failure.

A FAIL OPERATIONAL autopilot is one whereby any failure is contained within the system and the functions continue to operate. The downgrade due to an internal failure is to the status of FAIL PASSIVE. Should a further failure occur the final back up is the Pilot.

In order to conduct operations in CAT III weather conditions and to make use of CAT III airfield systems, and aircraft needs to have an Autoland system of either of the above types.

The typical weather minima with these systems are shown below for a UK airfield

As you can see there are a number of options to choose from and these are dependent on what the aircraft is capable of on the day. Assuming no qualification issues or airfield equipment deficiencies we can with a FAIL OPERATIONAL Autoland system (assume B787) fly down to a minima of 75m visibility and no decision height. Should some of the lighting for example be unserviceable there is a table we consult that points us to the lowest allowable minima on that table we can use given that failure.

If CAT III facilities are no longer available but CAT II and CAT I is available and the weather allows that (more of which in a bit) we can use this table.

A big part of FAIL OPERATIONAL autopilots is the presence of a certification requirement known as the ‘Alert Height’. This is a height that isn’t called out or made known during an Autoland approach but exists in a manual as a value. This typically is around 100ft above ground for Airbus and 200ft for Boeings.

The Alert height is significant in the handling of defects during the most critical point of the approach. Essentially below it, if we hear an alert from the aircraft we go-around, above it we can elect to go-around or handle any degradation by selecting a higher and more limiting minima should the weather conditions allow. For some failures only a go-around and reassessment of the aircraft’s capability is allowed. All of this is detailed in the relevant aircraft manuals.

The Alert Height is a certification requirement and must be above the MABH or Minimum Approach Break-off Height. This is a height from which, assuming normal handling characteristics and with zero reference to the outside environment, the aircraft should not touch the ground if a go-around is commenced. It is linked with the altitude loss from this height to commence the go-around and forms part of the certification basis for the minima approved for use by the state aviation authorities.

When assessing an aerodrome for CAT II/III operations, all of this will be taken into account by the airline and the authority in question. The airline will submit their proposal to the authority and it is they who will approve, or offer and amended authorisation for those operations. Part of this approval is demonstrating a company’s aircraft can indeed Autoland on that specific runway as we gain approval on a runway by runway basis, and not generically for an airfield. We often see notes on pilot briefing packages asking us to conduct one in good weather and report back on the success or otherwise of the approach and landing.

For example at JFK, we can not Autoland or conduct low visibility operations on all of the runways, the runways are neither equipped for the function and the approaches in use preclude it.

Some examples of types that have fail passive autolands are the B737-300/400/500 series and the EMB190.

Examples of fail operational aircraft are everything BA mainline currently operate. So all of the B777/787 and Airbus family aircraft.

Fail Passive aircraft can only operate down to CAT IIIA limits, and fail operational can operate down to CAT IIIB plus C albeit IIIC is not authorised or used in Europe.

The aircraft is constantly monitoring itself and will report at any point in the flight a degradation in its landing capability. For the majority of the time a Boeing wide body is capable of CAT IIIB operation and this is indicated by the lack of a NO LAND 3 message on the aircraft’s EICAS and the presence on the PFD of a LAND 3 below 1500ft radio altimeter height above the ground.

On airbus the PFD would indicate CAT 3 DUAL.

Should during the course of the flight an internal failure occur, depending on the nature there may be a system degradation to create a fail passive Autoland. This would be annunciated by the presence of the NO LAND 3 message on a Boeing and the LAND 3 would become a LAND 2. This signifies to the crew that they are now only able to proceed down to CAT IIIA limits at best.

With all of this in mind, lets see how the operation works in practise.
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Last edited by Sigwx; Jan 24, 23 at 7:13 am
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Old Jan 24, 23, 6:36 am
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We are going to break this down into two distinct parts, the departure and take-off and the approach and landing. Hopefully this section will answer some of those frequently asked questions that occur on foggy days.

Departure and Take-off

Weve arrived at Terminal 5 Heathrow on a foggy day with low visibility procedures in force, we have a morning departure and the airfield commenced safeguarding early on as radiation fog set in during the early morning.

The flight briefing pack is downloaded and start we going through a normal briefing but due to our knowledge of the required procedures and regulations, we brief a little more in depth.

Firstly, do we have an operation out of Heathrow? Heathrow and every other airport publish their approved take-off minima in terms of cloud base and visibility. For Heathrow it is 0ft cloud base and 75m of runway visual range. The current visibility today is 150m but has been fluctuating down to 100m.

Now any take-off below 400m of RVR is classed as a low visibility take-off and is a Captains only manoeuvre. Some operators use a higher value for the cut off and at BA this value is 600m for the First or Senior First Officer to handle the take-off.

Is our aircraft capable of doing this? Well with an operators approval to conduct low visibility operations, comes the training that allows us to take-off down to 125m RVR. With some specialist equipment that gives immediate information of the lateral deviation over the runway, and what to do to correct it, we can be approved down to 75m.

Our aircraft today is equipped with a head up display or HUD that contains a mode and display specific to this task of low visibility take-off or LVTO.

We check the operational defects and find nothing listed to prevent us from using this but we will double check with the aircrafts tech log once onboard.

In these conditions ATC at LHR will only allow us to start-up and push back once the RVR is at or above a certain value based on our capability. This is handily added to the summited ATC flightplan that was filled by the airlines flight planners.

The next question we need to ask is do we need to nominate a take-off alternate? We need a take-off alternate if we were to be unable to return and land at the departure airfield having suffered, for example, and engine failure on departure. Each state has its own prescribed planning minima for this purpose. For us today we can use the minima we are capable of assuming a limiting failure such as losing an engine. Thankfully for us we have approval to land single engine to CAT IIIB limits and that requires 75m visibility. The weather is better than that today with between 150-100m so we do not need to nominate a take-off alternate. To save time the flight planners do this for us, we just check it complies with the rules, but should we need one it needs to be within an hours still air flying time at the one engine out diversion speed. Typically this works out at around 300-400 nautical miles based on aircraft type. For an ETOPS approved aircraft, it is two hours, or 800 nautical miles give the rule distance that is approved for the operator by the state.

So weve got an operation out of Heathrow, but we know things will be slower paced than normal so we elect to carry some extra fuel for the extended taxi time.

Once at the aircraft we check the aircrafts tech log and prepare as we would normally until we get to the briefing. Some operators provide their crews with a handy aide memoir of the specific requirements and suggested areas to brief with regards to LVTOs and also approaches and landings. We will be discussing in depth the procedures Heathrow uses and the taxi routing expected, we concentrate on the HOWs of what we will be doing, how will we know we are on the correct taxi way, how will we identify the correct holding point? Who will do and say what for the LVTO, what will we expect to see and how will we know its all working as expected and how will we handle any failure? All of this is discussed until we are both happy of each others expectations and the plan.

The rest of our preparations are as normal and when we finally get clearance to start and push, we duly comply. Dependant on the actual visibility we will be taxing at a slower pace than normal. This is to ensure we dont get lost and the hardest part of operating an aircraft is indeed the navigation on the ground. On arrival at our nominated holding point we are looking for the maker boards, the wig-wams or inset amber flashers and the CAT II holding point ground markers along with an expected red stop bar that we absolute not cross until it is extinguished and we have a line-up clearance.

Heathrow will allow the CAT I holding point to be used down to 1100m and 300ft cloud base. As we are below this today CAT II holds are used and our set further back. We check that we have the ILS tuned for the lateral guidance we will use during the take-off roll. We will already have discussed the number of lights we will expect to see once lined up, and this will define the visual segment to confirm that along with the RVR given by ATC, we do indeed have the visibility required to commence take-off.

There is a regulatory take-off ban that exists until all of the following are satisfied;

The RVR as assessed by the commander is at or above the applicable minima (75m)

The reported touchdown zone RVR and/of met visibility are above this value

The mid-point RVR

The stop-end RVR is relevant (relevant meaning if entered at a speed of >60 kts should we reject the take-off at high speed)

The cloud base if relevant for the airfield in question (not relevant today)

Or is the runway and its markings are indistinguishable

We can only satisfy some of these once sat on the runway itself.

We are duly given line-up clearance, the red stop bar is extinguished and the alternating yellow and green taxi-way lighting illuminates, guiding out path onto the runway as we enter the ILS sensitive and critical areas.

From our briefing we reminded ourselves that Heathrow has centreline lights spaced just 15m apart, so to ensure a 75m visual segment, I must be capable of seeing 5 lights. As we passed through 45 degree to the runway centreline the HUD starts to come into its own, indicating where the centreline of the runway (according to the ILS localiser beam) is. We ensure we are lined up properly and that we have the relevant visual segment available. As clearance is received we are passed the three relevant RVRs and having confirmed they are at or above what we require, we commence our take-off roll. Should we have to reject our take off the lateral guidance prided by the HUD or the para visual display or PVD become very useful. The rate of deceleration on the 787 HUD is also displayed. It is also prudent for us to let ATC know that we are stopping. The ground movement and tower (runway) controllers sit in a visual control room, visual being the emphasis. In low visibility the visual part of that title is somewhat limited and so radar and transponder outputs become invaluable to know where the aircraft are. The back up to this are the standard RT or radio calls to ATC in these conditions. Whilst it is no longer mandatory to call airborne to ATC, we often still do this out of habit.

Once we are in the air and with having suffered no failures, thats that. We continue the flight as we would any other.

Whilst that covers a general overview of departing a foggy UK airfield, other stars such as the USA, Canada, Saudi Arabia and Australia have slightly more restrictive requirements and minima. Regardless all are in effect achieving the same end goal of a safe operation in conditions where our primary references are obscured, always remember that a take-off is a visual manoeuvre!

Nav Display with Airport Map Function and view of HUD on a standard day during preflight cockpit prep.

Approach and landing

Having noticed at the briefing stage that our destination is forecasting low visibility conditions, we have checked out NOTAMS to look for any unserviceable equipment on the airfield that might affect our approach and therefore the minima we are allowed to use.

With a fully serviceable aircraft we are consulting the page in the aerodrome booklet of charts that lays out the minima we have at our disposal. We can start to formulate a plan for landing. We consult the latest weather reports from the airport (ATIS) to see what we are capable of today and see that the visibility is around 250m.

From the table we can see that CAT II is not available to us, but we can do a CAT IIIB with no decision height (DH), a CAT IIIB with a 15ft DH or a CAT IIIA approach with a 50ft DH. The 200 in that box refers to 200m RVR, and 75 for the CAT IIIB and CAT IIIB no DH.

So to prepare for our approach we refer to our aide memoir and set up the aircraft for a CAT IIIB no DH as we default to the best available performance the aircraft can achieve to get the greatest chance of success on the first attempt.

Going briefly back to the aircraft and its capability, we need to be prepared for what is known as a reversion should a failure downgrade the aircraft to LAND 2 status. This will still enable us to operate to CAT IIIA limits. In this case then we can set up the radio altimeter to be able to make automatic call outs for a minima of 50ft. As we are planning on operating to CAT IIIB no DH we blank this display, but at the briefing we will discuss which single button to push in order to generate the calls outs should we need to revert and the weather still permits that.

The briefing is slightly more in depth than a normal briefing and again we refer to the aide memoir to keep it simple. We check we are both qualified to conduct this operation, that the aircraft is capable of CAT IIIB ops or plan on a lower minima. If we were to have an aircraft with a NO AUTOLAND message, wed need to find an airfield with CAT I weather or better as anything blow CAT I limits requires an Autoland. This is a company limitation and some airlines train for CAT II manual landings. We also make sure the runway is capable and discus any reversions that are available based on the actual weather and the minima available for that runway. We discuss the handling of various failures both above and below the alert height and how we will handle them. Some failures allow us to reconfigure and continue, some require a go around. As an example, if we were to suffer an engine failure below the alert height, we will continue with the landing, if the autopilot or auto throttle disconnects, we will go-around. We also discuss the runway exit and taxi route and any airfield specific procedures and what we will do with aircraft lights. To aid the visual requirements it is often prudent not to have the landing lights on in low visibility to reduce glare.

We now discuss the regulatory requirements for lighting we need to see in order to let the aircraft land itself. The reason for this is that we will need to disconnect the autopilot at some point on the runway and be prepared for it to disconnect itself in the event of a failure after touchdown. For a CAT II landing down to 300m visibility we need to see 3 consecutive approach or runway lights including a lateral element such as a touch down zone barrette. For CAT IIIA we just need 3 lights with no requirement for a lateral element and then for CAT IIIB it is at least 1 centreline light and for Cat IIIB with no DH we dont have to see anything at all.

So if we can land blind why the 75m requirement? We will need to taxi the aircraft with the human eye and therefore 75m has been established as the lowest visibility that can be safely conducted with the Mk.1 eyeball.

Weve now briefed and discussed the HOWs with regards to executing the brief and it is time to commence descent the approach. Autolands are the preserve of Captains as the pilot flying for the landing itself. Most airlines adopt a concept known as monitored approaches for low visibility operations. This means that the First Officer flies the descent and approach down to the minima for that approach at which point the captain will call LAND (or words to that effect) and assume control from the automatic or manual (if permitted) landing. The First Officer is at all times on the approach planning to do one thing only, go-around. They will deliver the aircraft in the correct landing configuration and in a stable condition by the prescribed point specified by the airline for the captain to land. The captain will however wait until the required visual reference (the number of lights required) has been achieved.

To commence the ILS approach we check the correct code for identing the system is displayed and that the lateral and vertical deviation pointers are working in the correct sense. As we are cleared to intercept we select the LOC or localizer mode of the Auto Flight Function suite. The VOR antenna in the tail will lock onto the localiser beam an align the aircraft to track the signal to the runway, as alignment finishes the LOC mode switches to the antenna in the radome for tracking. We arm the glide slope mode via the APP or Approach button and the glideslope antenna, also around the radome will intercept and track the glideslope beam. At 1500ft off the ground the autopilot engages all available channels, separates power sources and conducts a BITE check. At this point we see either LAND 3 or LAND 2 and we can now confirm the weather and minima we are able to use and any reversions that still remain. We are now approaching a critical regulatory moment for a few reasons, 1000ft above ground. The approach bad is now relevant. The regulations permit us to commence an approach more of less regardless of the reported RVR, but we are not permitted to go below 1000ft unless the RVRs provided by ATC in the tower are at or above the minima we require to land. Assuming we have values above this figure we can continue right down to our decision height. 1000ft also marks a critical point in the stable approach policy. We must now be in the correct landing configuration of flaps and gear and be in the correct vertical and lateral slot in the sky and at the correct stable final approach speed. If we are not we go-around. With everything in order the call of Stable, Autoland, I have control comes from the left hand seat. The Captain is now the pilot flying for their autoland. This role reversal is known as the monitored approach, and means one pilot is always go-around minded and ready to react if needed.

We are hoping there will be no system downgrades, but we are briefed on what to do for a variety of scenarios. The RVRs are behaving and its now prudent we know the that the touch-down zone RVR is always controlling, the mid-point value is controlling if reported at relevant and needs to be at least 75m, the same as the touch-down zone value today. Otherwise it would be 125m required there. The stop-end value will be a minimum of 75m and is only controlling if reported and relevant. For relevant, it is relevant if wed be in that segment of runway at or above 60 kts.

ATC have by now cleared us to land and we are given RVRs of 150/150/125, so we should actually see some runway today. We approach 100 and the 100 ABOVE call from the GPWS system tied to the radio altimeter.

With the LED lighting installed these days and the visibility better than required, a visual call is made by the Captain above the minimums call-out. Implicit in this call by the Capatain is the hand over of control and the controls are now guarded by the captain as the aircraft continues down. The FLARE auto pilot/flight director function activated and the thrust levers roll back to idle. The aircraft settles down. On 777s the para visual display (where installed) provides roll out guidance and the HUD on the 787 continues to show the localiser centreline. With the patchy nature of fog the visibility goes up and down as we roll and decelerate along the runway and at around 60 kts the captain squeezes on the top of the rudder peddle to take over from the auto brake and disconnects the auto pilot with its distinctive alarm.

The 3/2/1 count down lights of the rapid exit taxiway are seen and we can pick out the alternative green and yellow centre line lights. We follow these off of the runway and use ATC instruction, viewable taxiway signage and our charts backed up by the airport map function of the navigation display (where installed) to navigate to our parking position. A concept of follow the greens comes into its own here with clear red stop bars that help us to navigate correctly in what is the most difficult part of the flight.

So there you have it, what you don't know now isn't worth knowing. Along with Heathrow Tower's great post on the ATC side, you can hopefully begin to see exactly why we don't mess about with Low Visibility and why not all aircraft, runways or Pilots are capable of performing auto lands and low visibility take-offs, and just why extra spacing is required and the amount off effort that goes into maintaining an operation, albeit reduced compared to a lovely sunny day.

* For the Mods - As per HeathrowTower 's suggestion on the Cold weather thread, perhaps these could all be merged? *
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Last edited by Sigwx; Jan 24, 23 at 10:04 am
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Old Jan 24, 23, 6:36 am
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Why are there delays in fog?

I need to redo the images as they were hosted on Photobucket which ended up charging.
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Old Jan 24, 23, 6:47 am
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Originally Posted by Heathrow Tower View Post
Why are there delays in fog?

I need to redo the images as they were hosted on Photobucket which ended up charging.

Good reminder there Heathrow Tower I had forgotten this had been posted.
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Old Jan 24, 23, 11:43 am
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Another awesome piece of work by sigwx. Thank you!
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Old Jan 24, 23, 1:42 pm
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I might have missed while I skim read a very accurate detailed post on low vi's ops but didn't see a para on SMCGS. Not all airports have it, the taxiway centelines are lit like a trail of breadcrumbs guiding the pilot to the runway or terminal.

fun fact, the ATC at Aberdeen is shaped so as not to impinge on the runway low vis cone. Or was 20 years ago when I was on the ramp there.
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Old Jan 24, 23, 4:22 pm
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Very interesting, thanks for writing!
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