Performing the critical altitude test.

[Will.iam]

Looking for lessons learned or any tips on performing the critical altitude test. My airplane will be ready for pickup out of annual this thursday but in order to verify adjustments made to the turbo system i need to fly the airplane up to critical altitude to make sure ny fuel flow is set correctly. I did a flight to 22,000ft one day and noticed my boost pressure falling back to 30” pressure the A&P suggested after finding no leaks in the air intake system that the fuel flow could be dropping off and that certainly looks like the case as pictured below my fuel flow went from 24.7 down to 18.3 by the time i got to 22,000 and you can see the gradual drop off from my savvy chart circled in red. Just wondering also best way to come back down quickly without shock cooling or taking forever. I’m thinking of lowering the gear before the descent so i can keep up more power to fend off cooling the motor too quickly.  That and leaning mixture to peak egt on the start down. I’m also hoping that with the slope starting down at approximately .3 gal/hr less per 1000 ft climbed i might be able to tell within the first 5000ft or so as A&P makes fine adjustments until it holds steady up to 5000 then go on up to the FL to verify its able to hold 36” MP and the fuel doesn’t drop down in the climb  

[anthonydesmet]

What type of airplane you flying?  M20K?  252?  231? Watergate, inter cooler?

i take it no speed brakes.

[anthonydesmet]

Sorry, just saw your profile.  Didn’t come  up on my phone…..

I would crack the throttle 1inch from cruise, bring the rpm back to 2200, speed brakes out, cowl flaps closed and trim to 150/155 IAS…..should give you 1k/min…..if you are running LOP just leave the mixture….

[Will.iam]

For any of the 252 / encore pilots out there that fly up into the FL’s does your fuel flow reduce any as you climb to altitude? After checking intake manifold at my last annual they found no leaks in the system so their next focus was instead of thinking i was losing boost and the fuel pump was adjusting accordingly for less air going into the engine they are now thinking that I’m losing fuel flow to the point that it starts effecting my turbo to maintain enough boost pressure to keep 36” MP. They went through Continental fuel set up procedure. But the take-off fuel flow is closer to 27 gals instead of the high side of 24. I can lean it out to get back to 24 and make take power the interesting thing is as i go up into the teens i see the fuel flow start to lean out to 22 gals per hour even though my MP is still 36 but now i can push in the mixture and get back to 24 gals which still holds 36” mp but my cht’s come back from 390 back to 360 obviously cooling with fuel. I think it would make it to critical altitude with this setup but just trying to think why the mixture is not adjusting more automatically and keeping the fuel at 24 throughout to climb. Any thoughts on what could cause this to happen?

[Z W]

I would suspect the altitude-compensating feature on the fuel pump and your engine set up procedure.  A local A&P had ours all screwed up.  Maxwell's crew straightened it out at last annual and now it's great.

I would not leave it at 27 GPH at takeoff power.  You may be so rich the engine is not making full takeoff power if you forget to lean, or if someone else gets in the plane.

I've never noticed a fuel flow reduction in the climb.  I'll look for you next time I get a chance and am headed up high.  I do get a high fuel pressure alert at full power climbs somewhere above 15,000 feet.  I believe it's because the pump is set a little rich for takeoff, 23 GPH, which is recommended for cooling and CHTs.  So, I've started just reducing power to 34" MP or so when that happens, which is plenty of power for the last few thousand feet of climb.  

I don't understand the engine set up procedure well enough to say which screw needs to be turned on the pump, but MP, fuel pressure, and FF are all interconnected in there, and if any one is off, the engine does not run right.

[LANCECASPER]

1 hour ago, Will.iam said:

For any of the 252 / encore pilots out there that fly up into the FL’s does your fuel flow reduce any as you climb to altitude? After checking intake manifold at my last annual they found no leaks in the system so their next focus was instead of thinking i was losing boost and the fuel pump was adjusting accordingly for less air going into the engine they are now thinking that I’m losing fuel flow to the point that it starts effecting my turbo to maintain enough boost pressure to keep 36” MP. They went through Continental fuel set up procedure. But the take-off fuel flow is closer to 27 gals instead of the high side of 24. I can lean it out to get back to 24 and make take power the interesting thing is as i go up into the teens i see the fuel flow start to lean out to 22 gals per hour even though my MP is still 36 but now i can push in the mixture and get back to 24 gals which still holds 36” mp but my cht’s come back from 390 back to 360 obviously cooling with fuel. I think it would make it to critical altitude with this setup but just trying to think why the mixture is not adjusting more automatically and keeping the fuel at 24 throughout to climb. Any thoughts on what could cause this to happen?

Are you leaning in the climb??

The way I flew my Encore was throttle, rpm & mixture full forward, cowl flaps open, until I leveled off for cruise.

[kortopates]

Absolutely no reason to check critical altitude for fuel system setup. Fuel flow follows MAP period. The system doesn’t know what altitude you’re at either - only the upper deck pressure.


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[Will.iam]

59 minutes ago, kortopates said:

Absolutely no reason to check critical altitude for fuel system setup. Fuel flow follows MAP period. The system doesn’t know what altitude you’re at either - only the upper deck pressure.


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I agree completely the problem is as you look at my original post my fuel flow slowly goes down BUT my MP stays at 36” pressure! So my system the fuel flow is not following my MAP. I know my jpi data doesn’t show MP but i watched it in that climb stay at 36” until the ff was down to about 21gal per hour from that point on which was about 16,000ft the MP started dropping as i climbed until i got to 22000ft at which point i was at 30”mp with only 19 gals per hr and this was everything full forward. Since the A&P has set a richer setting as i go through the teens and the fuel ff starts dropping i can add more fuel with the mixture which brings the power back up and increases the MP back to 36” so for some reason my fuel pump is reducing ff like it has an altitude compensation feature when it should not because my MP is steady at 36” until the ff drops off so much that I’m losing power to turn the turbo enough to produce the required 36”.