mduell

2) You can air start most aircraft in commercial service, including up to 777/A380; much like the famous quote in Jaws, it's just a matter of "we're gonna need a bigger boat airstarter". Notable exception would be the 787 (electric start) and maybe the 747-8 (not sure if they backported an air start to it).

3) Yes, if you keep up the airspeed so they're windmilling.

4) There will be operational limitations, like prohibiting overwater flight, to mitigate the safety risks of an inop APU.

TomMM

737 manual start

zeus2120

That's some bravery performed by the ground worker right there.

LarryJ

Not very often. I've had two airplanes with inoperative APUs this year which is way above average. The most recent, the APU failed just as we were preparing for push-back. The mechanic read the fault codes and it was for a high oil temperature which caused the APU to automatically shut down. This could have been either an actual high oil temperature issue or a bad sensor. The ability to operate without the APU got us on our way about 30 minutes late instead of what likely would have been several hours, at best, if a replacement airplane had to be flown in.

An inoperative APU will be routed to maintenance as soon as possible as it is quit disruptive to the operation due to the extra time the air starts take. The MEL would specify the maximum number of days that the aircraft can be operated before the repair is made so indefinitely is not an option.

Yes, any size aircraft. Some aircraft may need larger air carts than others.

Not often. That's part of the problem. They are rare enough that most of the ramp workers have never done a real one which adds to the time it takes to get it done. They've been trained in the procedure but it never seems to go quickly and smoothly. You also tend to find air carts that won't put out enough pressure. They run fine until you put the load of a starting-engine on them but then they can't maintain the required pressure.

You don't need the APU to start an engine in the air because the airflow through the engine provides the needed rotation.

The APU is not normally used inflight though it is required as a backup for ETOPS and overwater flights. An aircraft without a working APU would not be dispatched on such flights. You also couldn't do bleeds-off takeoffs or landings so there might be a performance penalty in hot/high conditions.

The real issue with inop APU is trying to maintain cabin temperature. They have to disconnect the ground pre-conditioned air (PCA) before we can do the air start. That leaves the cabin without any conditioned air until an engine is started. If there are problems with the air cart you're stuck without air while they try to fix it or find another air cart. Meanwhile, they can't reattached the PCA if the jetbridge has been retracted. If the outside air temperature is much above 50, or so, the cabin will get uncomfortably warm.

Once you have started one engine, you start the second engine using air pressure from the operating engine. You can't do that in the gate area because you have to increase thrust on the operating engine to get enough air pressure and that jet blast would create a hazard for the workers and equipment around the gates.

That's a different problem. That is manually operating the start valve which is necessary when the start value actuating system fails. You have air pressure from the APU but the start value doesn't open electronically. I can't remember ever having to do one of these on a line trip.

findark

Interesting. I was on a 763 out of OGG last year where I was pretty sure we had an inop APU. From what I remember, they were reluctant to board but as the delay rolled, the captain came out and told us we were going to give it a shot, but there would be no cabin air while we were on the ground, so the plane would get hot and stuffy really quick (honestly wasn't thatbad). They spun up one engine (I don't recall if there was a cart) and then we rolled out to a pad near the start of Rwy 20 to get the other engine going, before taxiing back to take off on 2. After that we took off and flew to DEN.

Could that have been something else, given we were allowed to fly overwater? Or does it have to do with the 767s having a RAT?

LarryJ

That sounds like the APU was working but the APU bleed air was not. It is the APU generator that is needed for ETOPS, not the APU's bleed air.

The 767 RAT is only used if both engines fail. It provides hydraulic pressure to operate the flight controls. (I flew the 757/767 briefly at a previous airline). Here's a picture of a deployed 767 RAT in the maintenance hangar. Not a UAL airplane or hangar but it would look the same.

thesun

That's interesting - I was flying out of BOG a few years ago on a 757 with inop APU and there was some trouble in finding two carts needed to start the engine, and then some doubt as to whether the ones they found would put out enough air pressure to get the engine started.

At least we didn't have to worry about cabin temperature. Despite the evening temperature being 50F every day they still managed to make every aircraft feel like 90F nearly every time at BOG so it didn't actually get any hotter while we were hoping they'd get the engine started!

JimInOhio

An air start would be using a high pressure hose with super-heated air to get the engine going. I remember watching the groud crew outside my window do this to a plane I was on (probably a 737).

clubord

There are some aircraft maintenance differals that I would not hesitate to fly with domestically or even some ETOPS segments to Hawaii or Europe.

South or Central America, no way!

The resources and the quality of maintenance support staff/equipment is so limited you can easily get stuck there. The availability or reliability of huffer carts is a great example of that.

DCA writer

I had a flight out of DCA in January delayed because first our 737 had an inop APU, then the UA cart was broken, then they had to borrow one from DL, then the snow picked up (here I will gloss over the next several hours spent in the UC upstairs), and when the rampers finally connected the borrowed cart and fired it up, it spat out enough black smoke to make me wonder if that thing ran on coal. Is that normal?

(That did get the engine to turn, allowing one of the only post-8 a.m. departures out of National that day.)

ContinentalFan

I�ve been watching YouTube documentaries on incidents over the years. It�s impressive how safe flying has become. One thing that intrigued me is the disappearance of navigators in the 60s and 70s and flight engineers in the 70s and 80s. Technology made them redundant. What happened to them? For United lag them all off as aircraft requiring them were retired? Were they retrained for other roles?

One follow up question: as aircraft get more sophisticated, what are your thoughts on pilots being made redundant next?

LarryJ

Navigators were well before my time. Most of the professional flight engineers aged out as fewer aircraft needed them. Some made the transition to become pilots but that took a significant amount of effort on their part. Most didn't. Airlines stopped hiring professional flight engineers years before the airplanes went away. They started hiring pilots and having them serve as Second Officers, doing the FE duties, until their seniority allowed an upgrade to a pilot seat. This reduced the number of PFEs still flying when the airplanes were retired.

We are not even close to that point.

ContinentalFan

Thanks for the feedback.

fumje

If we can't get a Tesla to reliably distinguish a stop sign from a bank sign, we're not ready to have pilots made redundant. And personally, as someone who likes to have successful takeoffs and landings, I'm quite happy to continue chipping in to pilots' pay!

goodeats21

Thank goodness.