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Old 08-29-2018, 12:30 AM   #1
jetstorm96
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Default Failed to restart APU after shutting down

Dear ED Forums,

I come bringing another question, which may have a simple answer & I do hope so.
Ran a flight with the Mi-8 to the top of the snow covered mountain which has the static P51 sitting on the top.

Shutdown, then attempted to restart. The apu would kick the temp to up to normal and then just drop back to zero with a failed start.

Batteries looked like they were providing power.
I think the altitude was around 4 - 5000ft.

No matter what combination I tried, "Heating etc" I could not get the apu to start.
Any ideas what causes this?

Attempting an Autostart kept showing the message "power supply failure" - but I have seen this a bit and can bypass it on the ground.

Any ideas?
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Old 08-30-2018, 09:36 AM   #2
NOALB
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Yeah, the APU will flat-out refuse to start above a certain density altitude. I did some testing and found that the APU will refuse to start above 4422m ASL (true as taken from the F2 external view) @ 20°C & 760mmHg QNH as set in the editor (OAT read about -9°C). My altitude off the altimeter was about 4366m (with the Kollsman window set to 760mmHg).

I've wondered about that question myself, and I've tried just about everything including running all of the anti-ice systems, repeatedly trying to start the APU, running the КО-50, switching on only the essential CBs, using ground power, etc. It seems that the only way to achieve proper conditions for starting the APU is to be within the temperature and density altitude limits. And it's a HARD limit; the APU would refuse to start in my test if I moved the starting point just 1m up the slope (the limits are also affected by temperature and QNH changes). I wish I could provide a formula for finding the density altitude using the OAT gauge and altimeter, but I can't figure that out right now, lol

Only advice I can give is that if you need the engines running at very high altitude, either don't shut them off after starting at a lower altitude or use the hot start option in the editor. I would certainly like to be wrong about this (if anyone else can provide info) as it's been a minor annoyance to me as well.
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Old 08-30-2018, 10:38 PM   #3
Chic
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Quote:
Originally Posted by NOALB View Post
Yeah, the APU will flat-out refuse to start above a certain density altitude. I did some testing and found that the APU will refuse to start above 4422m ASL (true as taken from the F2 external view) @ 20°C & 760mmHg QNH as set in the editor (OAT read about -9°C). My altitude off the altimeter was about 4366m (with the Kollsman window set to 760mmHg).

I've wondered about that question myself, and I've tried just about everything including running all of the anti-ice systems, repeatedly trying to start the APU, running the КО-50, switching on only the essential CBs, using ground power, etc. It seems that the only way to achieve proper conditions for starting the APU is to be within the temperature and density altitude limits. And it's a HARD limit; the APU would refuse to start in my test if I moved the starting point just 1m up the slope (the limits are also affected by temperature and QNH changes). I wish I could provide a formula for finding the density altitude using the OAT gauge and altimeter, but I can't figure that out right now, lol

Only advice I can give is that if you need the engines running at very high altitude, either don't shut them off after starting at a lower altitude or use the hot start option in the editor. I would certainly like to be wrong about this (if anyone else can provide info) as it's been a minor annoyance to me as well.
Nice work.
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Old 08-31-2018, 11:20 AM   #4
AlphaOneSix
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As per the flight manual (the real one) the maximum altitude for starting the APU is 4000m. Take from that what you will.
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