N2 at high altitude airfields

[BrianCoyote]

Hey there, I was wondering if this was normal behavior. Pushing back and starting Eng 2 I noticed the N2 started falling off at around 20%. I usually introduce fuel at 25% but had to do so much earlier because the N2 wasn't spooling as high as fast as it usually does. This is a first for me and I have a feeling it has something to do with the airfield elevation being 7,300 feet MSL. Do the engines behave differently at higher elevations?

 

thanks

[Litjan]

It is actually the APU putting out less air pressure at high altitudes - and sometimes people forget to turn off the packs before they start the engines, too...

[BrianCoyote]

Must've been the APU pressure. I had packs off and all, then remembered I was at 7,300 MSL. So in that kind of situation would they bring out external air? Seems like it would make sense to.

[Litjan]

As long as you can get over 20% N2 you are ok. It is the lower limit to introduce fuel. I bet that the engine would still start OK even at 18% - you just need to monitor the start even more closely for a possible hot-start. But of course I would never do something like that, as it is against procedures . I would rather cancel the flight (if they have no air-start-unit) and send all 130 passengers into some shady hotel in whatever nice area you are that has a 7,300 MSL airport .

Cheers, Jan

 

[BrianCoyote]

4 hours ago, Litjan said:

As long as you can get over 20% N2 you are ok. It is the lower limit to introduce fuel. I bet that the engine would still start OK even at 18% - you just need to monitor the start even more closely for a possible hot-start. But of course I would never do something like that, as it is against procedures . I would rather cancel the flight (if they have no air-start-unit) and send all 130 passengers into some shady hotel in whatever nice area you are that has a 7,300 MSL airport .

Cheers, Jan

 

Mexico City https://en.wikipedia.org/wiki/Mexico_City_International_Airport

[Litjan]

Yeah, thats what I figured. Flew there quite a few times in our 747-400s. We always started one engine at a time when there (usually its two simultaneously) because of this very issue...

Cheers, Jan

 

[AndersenBali]

I'm glad I found this post.. I had a a departure from Kunming China at 6900 MSL and starting the engines was very different this time.. Spooling up to 22 pct took lot longer time than normal.. I noticed also during take off N1 and N2 didn't reach more than 80 pct after TOGA even though it was set to 93 pct in the FMC.

 

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[AndersenBali]

I'm glad I found this post.. I had a a departure from Kunming China at 6900 MSL and starting the engines was very different this time.. Spooling up to 22 pct took lot longer time than normal.. I noticed also during take off N1 and N2 didn't reach more than 80 pct after TOGA even though it was set to 93 pct in the FMC.
 
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I would like to add to this that starting the engines now at an airport at ie. 50 MSL is still not as it used to be.. Its like the ixeg 737 somehow had stored the old altitude in the system somewhere.. I hope what I'm pointing out here makes sense

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