CO764
Topic Author
Posts: 196
Joined: Sat Jan 21, 2012 5:50 pm

Long-range Flights Flying Higher/lower Altitudes.

Tue May 29, 2012 4:40 pm

Hi all,

Just a quick question :
Why do some long-range aircraft (eg. transatlantic flights) such as the 757-200, 767-200, 777-200, A330...etc. have a main cruising altitude which is initially sometimes as high as 40,000 feet while some aircraft such as the 767-400ER, 757-300...etc. sometimes never reach an altitude of even 32,000 feet?

The other day I noticed an AA 777-200ER from LHR to JFK reach 40,000 feet just 30 minutes after takeoff!
I assume the difference has to do with engine performance but can someone confirm?

Thanks,
CO764
 
jonnyclark
Posts: 106
Joined: Tue Feb 01, 2011 10:52 pm

RE: Long-range Flights Flying Higher/lower Altitudes.

Tue May 29, 2012 6:29 pm

Based on my commercial ground school pilot training, I would say it's less based on aircraft type and more based on conditions during the day, fuel load/payload, time in the air, and air traffic control clearances. I'm not sure, but logically, sometimes the smaller lighter planes have less payload, so the pay-off for travelling at higher altitudes may be better than those widebodies whose payload is larger and therefore fuel load is much larger. But that said, if the winds at FL320 are better for fuel consumption than the winds at FL400, even the smaller jets would fly at a lower flight level.

Although, the higher you go, 'in effect' the faster you go relative to Mach number too. This is all now calculated by flight planners with computer technology, so sadly, it's never really going to be a clear cut answer.
Jonny, commercial pilot & founder of Thedesignair
 
CPDC10-30
Posts: 4681
Joined: Wed Feb 02, 2000 4:30 pm

RE: Long-range Flights Flying Higher/lower Altitudes.

Tue May 29, 2012 6:54 pm

As mentioned, there are many other factors at play other than the aircraft design. But all other things being equal, my impression is that aircraft with a lower wing loading (such as the A330-300) tend to have a higher initial cruise altitude than those with a higher wing loading such as the 777-300ER even on similar stage lengths. I've noticed this myself and a cursory check of data on Flightaware seems to confirm this. AC's A330-300s will tend to start out at FL370 YYZ-LHR and climb to FL390. The AC 77Ws will generally start out at FL330 and climb to FL350 or rarely FL370.

I was once on a EuroAtlantic 767-300ER with only about 30 pax for LGW-YYZ (2nd flight of the year for SunWing). I figured it would zoom right up to at least FL380 based on the light loading. But it stayed down at FL340 for the whole flight. There must have been a good reason  
 
KAUSpilot
Posts: 1659
Joined: Mon Jan 28, 2002 2:15 pm

RE: Long-range Flights Flying Higher/lower Altitudes.

Wed May 30, 2012 7:53 am

Weight.

In the 744 we will typically only climb to the low 30's if we are near max takeoff weight. The airplane is not capable of safely flying much higher at those weights. Sometimes it doesn't make sense to climb later in the flight either, so we just stay in the low 30's. Wind, weight, flight duration and fuel temperature all play a role in that decision. The flight plans are generated by advanced software which tell us exactly when and where to climb for best efficiency, taking into account all variables. If we are empty we will usually climb to FL 400 or so right away. Also, on North Atlantic tracks sometimes it's not possible change altitudes once you've begun the crossing. By the time you get to the other side, it's not efficient to climb anymore.

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