Engine "Core Lock"

[Oilhead]

Anyone here know anything about this topic, or experienced it?

[barit1]

The only phenomenon I know like this -

When rotor/stator clearances are tight (as in an engine fresh from the shop) the clearance may shink to zero after shutdown. This is because the lightweight case cools off faster than the massive rotor discs.

It may be impossible to crank the engine when this happens.

The situation resolves itself as the temperatures slowly equalize.

[Milt]

Good Grief!!

Let's hope that any engine that "Siezes"because of inadequate clearance between rotating and stationary parts gets an instant reject especially if some attempt has been made to crank. A bit like trying to crank your car engine immediately after a seizure.

There was a similar problem over clearances with Avon engines in the RAAF's F86 Sabres. Many would vibrate excessively in the climb through medium altitudes and enough for pilots to reject. No amount of coaxing could reproduce the vibrations on the ground and many engines were completely overhauled at great expense for no result

Elaborate flight test instrumentation and numerous flight tests eventually pinpointed the problem.

The Avon engine, and I guess most others, have what is called a balance piston as part of the rotating spool. Using compressor bleed air pressure on one side and atmospheric pressure on the other side the resulting imbalance is supposed to take out most of the end thrust on the spool bearings.

Trouble was the balance was too accurate under some conditions and the spool floated rapidly back and forth against the bearings.

The fix was to increase the clearances around the labyrinth seal on the periphery of the balance piston thus ensuring that there was always some residual end thrust on the bearings.

Have there been similar problems with other engines?

[Air Conditioned]

Cute name for a condition known on a number of engines. It is my understanding that the (or maybe one) cause is heat percolating to the top of the casings and the assymetrically warm rotor going out of round for a time after shutdown.

I have had Garrett TFE 731 LP shafts prove hard to turn during a brief turnround. On this engine there was an instruction that on a short turnround, of some specified duration, the fan had to be turned 90 deg before start. Given that the fan is geared down about 2:1 the inference is that the cool lower section of the rotor will then be at the top and temperatures will even out. That point however was never explained and this is my guess.

[LanFranc]

Does your question relate to the NWAirlink CRJ accident last December? I've heard that phrase being used in connection with that investigation. This is just what I've heard from people on the property, but they used the phrase you're talking about.

After the inflight stall and failure/flameout/shutdown the crew did not get the engines going again. Because of insufficient airspeed both CF-34's cores were not rotating at all (core locked) and contributed to them not being restarted in flight.

[barit1]

Is this the case? (http://www.showmenews.com/2004/Oct/20041021News014.asp)

[GotTheTshirt]

Milt,
This is not uncommon and is certainly not a rejection !

As Barit says usually happens on engines fresh from the shop especially when the 3 minute idle cool down is reduced.

Usually what happens is a very short run down then engine found the be locked up.
Manual recomends allow to cool and restart when engine is free obviously checking things like filter and MCD's.

Usually the areas causing the lock are self clearing ( Like tip seals ) and normally the engine will continue a long and happy life
:ok:

Have had it on 219 engines on initial runs and then no problems after that.

[Speedtape]

Also known as bowed rotor syndrome and affects the IAE V2500 series engines fitted to Airbus A320 series and some McDonnell MD90s. This is known to happen particularily after use of Reverse above idle followed by short taxi to stand and shutdown without allowing sufficient cool down to stabilise the core temperatures.

[barit1]

Bowed rotor is the convective temp gradient after shutdown. It can happen with any engine, and it's usually at its worst 30-60 minutes after shutdown (i.e. normal turnaround time :} ). If the engine is restarted at this point, vibs will be abnormally high for a few minutes. I suppose the bowing could be bad enough to cause a rotor/stator bind, although I'm not familiar with this occurring in real life.

In any case, while bowed rotor and core lock are related (differential cooling after shutdown), they are really separate phenomena.

milt said -

The Avon engine, and I guess most others, have what is called a balance piston as part of the rotating spool. Using compressor bleed air pressure on one side and atmospheric pressure on the other side the resulting imbalance is supposed to take out most of the end thrust on the spool bearings.

I suspect many different engine types have had this problem at one time or another. The fact that the Avon bearing thrust fell to zero is really a matter of bad luck on the designer\'s part. Designing just the right amount of preload into the bearing is a real crapshoot; there are so many variables in the game. Fortunately the fix action is usually fairly simple.

But bearing thrust load is a different problem from either core lock or bowed rotor.

[mcdhu]

I'm sure I remember a similar phenomenon called ''centreline closure'' on the RAF's Astazou powered Jetstream T1s. One of the implications was that when carrying out a premeditated engine shutdown - for training - said engine had to be idled for a minute or so beforehand thus giving the flumoxed student time to rehearse the event!

Cheers,
mcdhu

[A330AV8R]

In the case of IAE V2500s the SOP for not letting that happen is 3 mins of running after using reversers ..... taxi time is included in this .

TOGA SRS

[barit1]

In the case of IAE V2500s the SOP for not letting that happen is 3 mins of running after using reversers ..... taxi time is included in this .

That's good practice with any engine for best performance retention.

[Bill Smith]

Happens to PW123E engines as well, but is due seal failure between the 1st and 2nd stage turbine.

Symptoms of partial or total component failure are:

Significant engine vibration when selecting Condition Levers to MAX after engine start, coupled with vibration through the related Condition Lever.

No propeller rotation after stabilised idle Nh during start - described as Power Turbine Lockup.

A propeller which cannot be rotated by hand - Power Turbine Lockup.

A propeller which decelerates quickly after engine shutdown.

The symptoms described are caused by fragments of the failed seal or associated components lodging between the rotating shaft and seal housing. Information to date suggests that if a component is faulty, failure is likely to occur between 1100 and 1500 hours of engine operation. In all recorded cases, the internal seal remnants have been ingested by the engine with no catastrophic effect on engine operation. Operation with the seal missing has not caused secondary failures.

[3holelover]

G'day gents... I'll take a stab at adding to the answer:

There are undoubtedly specific examples of various engine types experiencing this because of specific anomolies, but generally, it's simply the result of the case cooling and shrinking quicker than the turbine rotor.
Clearances are kept to a minimum for efficiency. Some cases are cooled through the use of "Active Clearance Control"... Some (CFM and GE, for instance) use controlled methods of case cooling for both HPT and LPT case sections... - valves controlled hydraulically by the HMU or FCU that permit cooling air flow through the ducting to the case cooling manifolds as desired.

When the turbine rotor is still quite hot, and the case cools relatively quickly, the clearance can shrink to zilch. It's a bit difficult to envision "cooling" of the turbine when you're still pouring fuel into the fire, but obviously at idle for a time it'll be relatively cooler than it had been with some power applied. Usually, as has been said, the potential for this "lock" can be reduced, or is eliminated by observing the prescribed "cool down" period.

I've been told the most common intances of this occurring have been in heliowopwop aircraft with pilots who think it looks cool to come in hard and shut down right away.... They've had some power applied for their landing and the machine is hot.

Cheers,