B737 inflight engine start

[LEM]

In your opinion, is there a valid reason for this apparent contradiction:

the minimum pressure is 30 psi, decreasing by half psi every 1000 feet.
That's a non AFM limitation.

So why in the Inflight engine start NNC the minimum pressure is still 30 psi?
At 20000ft, 20 psi should do, according to the previous rule, also because the shaft is already rotating - or will rotate more easily - due to the relative wind, and also the differential pressure between the duct and the atmosphere il less.

:confused:


Another question: the Aborted engine start Recall Items don't seem to be applicable inflight, as the no-light up case and the apparent hung start case are covered by the cklist (still read and do, not by recall).

Only the Hot Start case is not covered.
Is it because a hot start inflight is almost impossible?
Has anyone got direct or indirect experience of a hot start inflight? (I mean in actual life, not in the sim where the boss can just push the hot start button if he likes to...)

[I-2021]

Only the Hot Start case is not covered.
Is it because a hot start inflight is almost impossible?
Has anyone got direct or indirect experience of a hot start inflight? (I mean in actual life, not in the sim where the boss can just push the hot start button if he likes to...)

Well, in case of inflight engine restart the max EGT is still 725 if one engine is already running. So it's like when you're on the ground.

[barit1]

What model 737 are we talking about? It may make a difference because of the implementation of the oil gage.

Engines I'm familiar with have a differential pressure oil gage - measuring the oil nozzle pressure vs. the sump environment. That way the oil trajectory is correctly assessed. In that case I don't think the indicated pressure should fall off at altitude.

[I-2021]

Oil pressure:confused:

[barit1]

I see now LEM is talking that OTHER fluid pressure. Makes more sense now, but he left me in the dark...

[CaptainSandL]

LEM,

Re 30psi for the in-flight start; my guess would be that a minimum of 30psi gives you the best chance of a normal start. With one engine still operating 30psi is no problem to achieve, so why not have it.

Re hot starts; fortunately no direct experience, but I have done about 30 CFM56-3 in-flight restarts this year, both starter assist and windmill, all from the edge of the envelope and I have not had a peak EGT much above 600C. Then again, I have done thousands of engine starts on the ground and not had a hot start either!

Not directly answering your questions but here is some good background info on the subject that I got from GEAE:

"Regarding the In-flight envelope:
The in-flight start envelope represents the conditions where a successful start is guaranteed from a stabilized windmilling condition as verified during certification testing. Successful starts may be accomplished outside this envelope (both with respect to airspeed and with respect to altitude), depending on the thermal state of the engine and the residual rotation speed; e.g. if a start is attempted while the engine is spooling down (called a "quick windmill relight”), the engine may successfully start at a lower airspeed or higher altitude than the published envelope. CFMI does not discourage airstart attempts outside the envelope but engine parameters should be monitored closely for an engine stall or EGT exceedance. Also, in the case of an all-engine out case, consideration should be given to the drain on the battery during start attempts outside the envelope which may have a low probability of success.

Maximum EGT.
The certified EGT limit for start is 725 deg C both on the ground and in flight. However, both CFMI and Boeing encourage the use of 930 deg C (takeoff EGT limit) to afford the pilot every opportunity to get a successful start in an all engine out situation. The higher EGT is not as severe for the engine in-flight as there is significant airflow through the engine unlike the static ground start case."

Barit1: the psi being referred to was duct pressure not oil pressure

S&L

[LEM]

but I have done about 30 CFM56-3 in-flight restarts this year
Hi Chris, you lucky guy seem to have a lot of fun! :D

[IFixPlanes]

...the shaft is already rotating - or will rotate more easily - due to the relative wind...... but the wrong shaft. ;)

[LEM]

Well, also the N2 shaft, a liiiiittle bit... no? :hmm: