High altitude operations (~ 8000ft)

[Dozo]

I have a question about high altitude operations.

Recently, I learned that the normal 737 (normal as opposed to one having the high altitude operation option) has a maximum landing altitude of 8400ft. Today I tried to autoland at SEQM, which is located at 7910ft. That didn't exactly go as planned as the aircraft bounced quite a few times down the runway. Manual landing was also quite a challenge and interesting.

Therefor, the question is: how accurate would X-Plane simulate less dense air at those altitudes and how much would I be pushing the boundaries of the IXEG 737 operation it in and around the Andes?

Curious for your thoughts and ideas.

[Litjan]

Hi Sebastian,

I think X-Plane captures the high-altitude aspect fairly well. It does not calculate the changing of true altitude with different temperatures, that is probably the biggest thing missing in mountain flying (remember the old saying "In the winter the mountains are higher").

The main aspects of high altitude operations is higher true airspeed (needs longer runways and wider turn radii in flight), and also less engine thrust and higher N1s than at "lower" airports. This is both modeled well.

The autopilots landing performance is tuned to sea-level airports and while it will land "safely" at higher altitudes, it may not land gracefully. Of course our autoland code is not as refined as the one in the real aircraft, but I can tell you that the real airplane doesn´t land beautifully with autoland quite often, either (both Airbus and Boeing). Its not like some super-pilot is kissing it at 1000 feet. Its more of a "playing it safe" landing...

Another aspect is starting the engines, you will see that the APU puts out less pressure at high altitudes, so reaching the desired N2 for fuel introduction takes a bit longer. In the 747 at Mexico City we could "only" start one engine at a time and not run a pack at the same time due to this effect.

Also do not forget to set up your landing elevation in the cabin pressure controller correctly. If you forget this, the plane will land pressurized (not allowed) and when moving the FLT/GRD switch to GRD after landing the pressure will be dumped immediately, creating a very high rate-of-climb in the cabin. Also remember that on longer flights TO a high-alt elevation you want to set the landing elevation to 6000 feet until starting the descent - that way the cabin altitude is not really high during the whole flight (may cause medical problems with the passengers). Yes, they will have to suffer the same thin air after they get out and have medical problems then - but then its not your problem anymore .

Cheers, Jan

 

[Dozo]

Hallo Jan,

Thank you very much for your elaborate answer to my question! You're confirming what I already suspected; high altitude operations are some thing else as opposed to 'regular' flying altogether. I did mind the landing elevation, but didn't know about the 6000 feet rule that would prevent me from having medical problems on my watch

See, we could use that cargo version - cargo doesn't complain as much

Vielen dank!

Edited July 12, 2020 by Dozo

[Litjan]

Yes, cargo doesn´t complain...but there is still a lot that can go wrong, especially when carrying lifestock...

Cheers, Jan