TP 331 Question

[boofhead]

TPE 331 Garrett Engine Does anyone know why the rpm restriction of zero time between 18 and 28% is required? What is the risk? What damage would ensue if ignored?

Reason I ask is because the NTS is set to hold the engine rpm at those ranges following engine failure. The limitation is posted for engine operating so would it not apply if the engine was windmilling (inop)?

[ThreeThreeMike]

A Honeywell document states the following regarding TPE331 engine starting:

CAUTION
DO NOT ALLOW THE ENGINE RPM TO “DWELL”
BETWEEN 18 AND 28 PERCENT RPM. IF NORMAL ENGINE
ACCELERATION WITHIN CRITICAL SPEED RANGE
DOES NOT OCCUR OR IF THE RPM STOPS INCREASING
PRIOR TO REACHING NORMAL IDLE, “ABORT THE
START”. (Refer to AFM/POH)

This passage is found in the section regarding air starts:

- It is important not to allow the engine to windmill between 18% to 28% RPM (shaft critical RPM) on TPE331-1 through -12 or between 14% to 20% RPM on TPE331-14/-15 engines.

CAUTION
CONTINUOUS OPERATION/WINDMILLING WITHIN THE
SHAFT CRITICAL RPM RANGE MAY CAUSE ENGINE
DAMAGE.

Thus the limitation apparently applies for all modes of operation. The mention of a 'critical speed range' and 'shaft critical RPM' indicates the issue is likely related to resonance.

http://eastaire.us/files/TPE331pilotnotes.pdf

[glendalegoon]

I flew the garrett on a couple of planes, the MU2 and the SW Metroliner

its been well over 26 years, so here goes on my fading memory.

During start, dwelling in a certain range is almost always caused by a BOWED SHAFT due to uneven heat dissapation during shut down

we use to pull the prop through a number of times after shutdown to avoid a bowed shaft for subsequent starts.

IF the engine would not accelerate through this range we gave up on the start and sometimes would go out and pull the prop through a number of times to get rid of the heat induced bowing.

but then again I could be wrong

[ThreeThreeMike]

Glendalegoon, in the document I referenced above there is a section regarding shaft bowing. It mentions the procedure you recall, rotating the engine by hand to verify the shaft is free.

[glendalegoon]

it also allows cooling air to help in preventing the bowed shaft

the garrett is a good engine...but real jets are even better!

[westhawk]

I used to do a little mx work on the TPE-331. (turbo-prop engine?) Mostly as installed on Metros and Jetstream 31s but some others too.

Yes, rotating the prop by hand shortly after shutdown is done to minimize the effect of shaft bow by ventilating the engine with cooler air. As I recall, the primary effect of shaft bow is excessive abradeable seal wear on the next start. And IIRC from the Garrett TPE-331 line mx school in PHX 22 years ago, resonance is indeed the concern WRT the restricted RPM range. I've seen similar restrictions on a couple of piston engined propeller driven planes as well.

Very efficient engines. Especially when measuring efficiency on a noise per pound of fuel basis! (well... at ground idle anyway... They quiet right down when you take 'em off the start locks and push the torque up a bit.)

[Metro man]

From memory this was due to engine vibration in this range. The TPE331 isn't the only engine to suffer this problem, the IAE V2500 (Airbus320) also has a keep out range. With the Airbus the engine computers won't allow the engines to stabilise in the zone which is one of the reasons why the thrust levers are partially advanced to ensure the engines match before setting take off power. If one engine sped ahead while the other one lagged you could be off the runway.

[Journey Man]

Ventilate or rotate

Yes, rotating the prop by hand shortly after shutdown is done to minimize the effect of shaft bow by ventilating the engine with cooler air.

Might be splitting hairs, but hand rotating the prop doesn't effectively ventilate the engine relative to the rotation speed even in the final phases of shut down, which is when most of the hot gases would be vented. Plus the number of recommended rotations is negligible in the grand scheme of things. (Three full rotations, if I remember correctly?)

Thermal bowing is at its worst around twenty to twenty-five minutes after shut down, as the warmer air has had time to rise, and cooler air to sink. Rotating the prop after about ten minutes mitigates the bowing by mixing the air again and leaving it sat in a different position. It's moot after approximately an hour as temperatures in the casing and shaft will have equalised. You often see eager crews leap out, spin the prop a minute or two after shutdown and think "job done". You can pretty much guarantee in a short turn around there is no further check of the engine for thermal bowing before starting again... I contribute this to the 'venting theory'.

[boofhead]

Thanks for the replies. Some useful stuff for me to think about. I was told the NTS cycles the engine between these numbers if the engine fails in flight and it seemed strange to me that they would set it up that way.

[harpf]

NTS

On the 331 after shop visit a test is required to check the NTS RPM. This done to ensure the engine windmills above 28%. The target is 32% as I recall. This done by shutting off the fuel and watching the the NTS set the prop pitch to an angle that result in 32%. NTS does not govern or controlled at any speed just puts the prop in compromise position. To reduce drag until the pilots secures the engine. The test is conducted at ~6000 MSL and 160 kts.