Making up for departure delays
 

There was a discussion of this in today's United system outage thread, but I wanted to start a new thread to avoid getting off topic there.

People were asking if it's possible for airlines to make up for lost time. The answer is yes, but they won't always do it.

Airlines usually run the planes below maximum speed to save on fuel costs. This is the same principle as in your car. Safety issues aside - suppose your car has a maximum speed of 120mph. You could drive 120 mph to get to your destination as fast as possible, but driving so fast causes more air resistance, and is below the maximum efficiency of your engine. If you drive at 60mph though, your engine is running more efficiently and you have less air resistance, giving more fuel economy. Same principle applies for planes.

However, if a flight is delayed in departure, the airline will sometimes fly the plane faster to get it to the destination on time.

When do they do this? Fortunately, we have a lot of data to look at this, and Nate Sliver at 538 has done just such an analysis here:

http://fivethirtyeight.com/features/...me-in-the-air/

Airlines try to get the plane to arrive close to the scheduled arrival time if possible, since on time arrivals are tracked and are an important performance metric. They also usually include a bit of extra time in the schedule for this event. So for a 6 hour flight delays of <30 minutes the flight runs as normal. For delays between 30 and 60 minutes, the plane runs faster than usual to make up the time. For delays more than 60 minutes, the are not going to make an on time arrival anyway, so they go back to running the planes at the slower, more efficient speeds.

Thanks for sharing this ^ and as this encompasses all airlines, I'll move this thread over to the TravelBuzz forum

goalie
UA Forum Co-Moderator

In addition to speed, there's also some amount of time that can be saved via routing optimizations -- back when it was more common to have channel 9 active on UA flights, I'd often hear the pilots requesting a direct to a particular waypoint, presumably bypassing another point. Depending on the level of traffic, the request would be granted or not.

Making up for departure delays
 

Don't forget there are other variables that are considered in setting the scheduled times but the facts on the ground at the time may change. For example, stronger tailwinds then normal will allow an aircraft, all else being equal, to reach the destination faster than anticipated. Amount of expected air traffic, time built in for taxiing and waiting for take off, etc. can all change based on specific set of circumstances. Some days, I land at ORD on a domestic flight and we are practically right at the gate. Other days, they use the runway out by the international terminal and taxi can be loooong. These factors are why different flights between the same origin and destination can have variable trip times, even when the same aircraft is used.

Making up for departure delays
 

These days with all of the advanced planning tools used in filling routes there are many times when pilots will refuse directs because it will actually be longer. DL and FedEx are the biggest that come to mind that don't always want to go off of the planned route on transcon flights. As far as speeding up, they do sometimes do this, but depending on the city pairs it doesn't always do that much. As an approximation, increasing mach number at cruise increases ground speed by about 7-10 knots per .01 mach (M.79 about 7 knots faster than M.78). A typical B737/A320 will cruise around M78 and very rarely do i see more than M80 or occasionally M81 (especially on long flights, burns to much fuel). Another way to increase ground speed at the expense of fuel is to stay lower; M78 at 30,000 is faster than M78 at 36,000 (ignoring wind differences).

I think the author of the article used wheels off compared to scheduled departure for departure delay (which it shouldn't be) and used arrival at gate to calculate flight time (which it also shouldn't be). To get accurate comparisons it should be based on Off to On for flight times (the out and in aren't usually anything the pilot can do) and really needs to account for time of day for taxi delays being different. everyone that's flown out of JFK during 'rush hour' knows how long the wait for takeoff can be as opppsed to the slack times during the day.

Still an interesting read, thanks for posting.

Dan

Another variable is winds aloft. Winds aloft are always from west to east, and the schedule is based on typical winds.

If either flying into headwinds less than forecast, or tailwinds stronger than forecast, you have a choice.
1 - Decrease power to maintain planned groundspeed (much more fuel efficient), or
2 Maintain the planned airspeed (higher groundspeed for the planned fuel burn).

It's an easy way to make up time for no fuel penalty.

Works opposite the other way. If higher headwinds or lower tailwinds than expected, it's very hard to keep on schedule no matter how hard you push the throttles.

Even if the plane can fly faster that doesn't always mean that it can land earlier at the destination. The calculations which go in to sequencing the planes in and out of airports may dictate that, even if the plane showed up 15 minutes early, that it would wait for 15 minutes to land so that it keeps its place in line.

UA is known to decline directs/shortcuts often esp. from the Bay Area going E. More likely to follow company dispatch's route planning, even if the flying time's longer than by using the shortcut.

too bad in the US there is no equivalent to the eu EC261 directive that define monetary claims for delayed flights.

its another variable on top of ota statistics and costs of rebooking. its a financial incentive that favors the flyers.

ive been on many flights on the eu that were delayed, but arrived barely just within the allowable delay window... which would probably be operated with less urgency if in the US

Here in China, you get assigned a landing at time x and arrive at time x (flying faster is of no use).