Mdthbs

I was on a flight from Philadelphia (KPHL) to Boston (KBOS), after some delay and some change of route due to weather, the captain announced that we will be cruising at 7000' due to delays in the area.

The history of the flight is here: https://flightaware.com/live/flight/AAL2606/history/20190618/2235Z/KPHL/KBOS

My question is two fold:

• Why cruise so low with such an aircraft (A319)?


• What is the impact on fuel consumption, knowing that we were also slower?

The only reason for your flight to operate at such low altitude is because it is cheaper for them to do so.

1) As you said it is due to weather, other route/altitude may not be available. They can cancel the flight but that is likely to be costly. They may have to find accomodation for you and crew until they can put you to the next flight. Sub-optimal flight maybe better than no flight in this case.

2) They may need that aircraft at that location the next day. If they choose to cancel your flight today, they will have to find a way to have this aircraft or another aircraft to fill in for the required flight the next day. They might as well fly a slightly more expensive flight at lower altitude rather than fly an empty plane to Boston.

3) It is possible for them to wait for weather to improve but there is an issue of crew working hour. A pilot cannot work more than 8 hours in a day without provision for additional crew. They may approach that limit and the wait for weather to improve might means the airline has to provide another crew for the flight and might mess up crew schedule for the next few days.

So for the airline, in this case, flying lower and slower maybe a better option for them. Hence higher fuel comsumption is better than no flight.

As you've seen from FlightAware, the route was longer because it was around the weather (turn on weather overlay if it is disabled).

I used the playback function of Flightradar24 for the 18th at 23:00 UTC, and the amount of traffic above 10,000' (filtering by altitude) seemed very normal compared to other days. I'm baffled as to why they flew so low, but I can address your fuel question in some detail.

The decision to go ahead and fly low, as has been covered, is not just about fuel. There is a myriad of direct and indirect costs associated with cancelling a flight, and its knock-on effects.

We can work out an estimate for the extra fuel from the A320-family performance manuals. First we need the distance flown, because as you said, flying slower is a factor (burning fuel for longer). According to FlightAware, the actual distance was ~360 NM (nautical miles).

The performance figures I have only go as low as 10,000', but it's close enough for an estimate. At a weight of 50 tonnes:

• At 10,000' each engine burns 1028 kg/h while doing 280 knots true airspeed (indicated airspeed is 242) taking a total of 1.29 hours.




• At 29,000' each engine burns 1305 kg/h while doing 462 knots true airspeed taking a total of 0.78 hours.

Therefore, the fuel consumption estimate per engine for this trip is 1326 kg (low altitude) and 1017 kg (high altitude). Having two engines means it's a total extra of only 600 kg. Using North America's jet fuel price of the week, that's $360.

It may seem low, but that can be a good profit on another short-haul flight (volume economics), but it certainly is not disastrous.

Possibly, the flight was too dangerous at a high altitude due to complications in weather, and therefore they decided to fly lower and slower. Flying at 7000' is very fuel-consuming, but they could do it since Philadelphia and Boston are fairly close together.