Thermal effects in aircraft engines - sudden/rapid cooling

[Astronuc]

I was watching some videos about aircraft performance and found a common theme - rapid or sudden cooling - of piston engine heads (there is some disagreement/controversy on the subject) or turbine stators, leading to component or performance failure.

The Focke-Wulf Fw 190 A-4 had a performance problem with the use of methanol-water injection (in the supercharger), a technique which was introduced in fighter aircraft of WWII.



The video references a Lycoming page on the subject of rapid or sudden cooling of hot engine parts.
https://www.lycoming.com/content/how-avoid-sudden-cooling-your-engine

https://en.wikipedia.org/wiki/Shock_cooling
https://en.wikipedia.org/wiki/MW_50

Rapid cooling of a cylinder head may result in thermal fatigue, crack initiation and subsequent propagation. On the other hand, over-pressure with respect to design could increase the stress to the point where crack initiation might occur.

The second effect, core lock, happens when a hot engine shuts off under cold conditions, in which the stator or housing cools rapidly and shrinks (thermal contraction) onto a hot rotor (shaft or spindle in the hot (turbine) zone). It is difficult or impossible to restart an turbo jet engine if the stator shrinks onto the hot turbine discs and/or shaft. This is was on aspect of a crash involving a CRJ aircraft that flew too high (upper service flight level of 41000 ft), at too steep an angle, resulting in both engines flaming out. The cooling of the engines at altitude locked the rotors such that the engines could not be restarted, and the plane eventually crashed.

https://en.wikipedia.org/wiki/Core_lock

Note: there are various issues with the Wikipedia articles. Nevertheless, the subject matter is interesting and should be appreciated by pilots and aircraft designers/engineers.

 

[Baluncore]

There are times when a cooling engine can be an advantage.
https://en.wikipedia.org/wiki/British_Airways_Flight_009
"As the ash entered the engines, it melted in the combustion chambers and adhered to the inside of the power plant. As the engine cooled from inactivity, and as the aircraft descended out of the ash cloud, the molten ash solidified and enough broke off for air to again flow smoothly through the engine, allowing a successful restart."

The thermal expansion coefficient of the rock, and of the turbine blades, were sufficiently different, that the bond between the two materials sheared as the engine cooled. That is not something that can be relied on.

 

[FactChecker]

This adds good detail to the fact that engine cycles more than flight hours are counted for engine maintenance scheduling.

 

[Vanadium 50]

engine cycles more than flight hours are counted for engine maintenance scheduling

Is that true? I thought it was X hours or Y cycles. (Of course, best of all is when some function of X and Y crosses a threshold)

Aircraft engine manufacturers have strong incentive to get this right, as their business model depends on maintenance. One told me that they sometimes even give the engines away for free - they make the money off the contract.

 

[FactChecker]

Is that true? I thought it was X hours or Y cycles. (Of course, best of all is when some function of X and Y crosses a threshold)

This is not my field of expertise, so maybe others can give a more authoritative answer. My understanding is that neither would be ignored but that engine cycles and excursions (full throttle, especially with afterburner, versus reduced throttle) are the main drivers.

Aircraft engine manufacturers have strong incentive to get this right, as their business model depends on maintenance.

Yes.

One told me that they sometimes even give the engines away for free - they make the money off the contract.

I didn't know that. Interesting.

 

[Vanadium 50]

I assume a "free" engine means you are contracting for an engine and N servicings, and this is split for accounting purposes. But I don't know - I don't do this for a living. The person who told me this was working on CFD and combustion modeling to better understand where and how engines break down. As I said, there is business incentive to get this right.

 

[Flyboy]

This adds good detail to the fact that engine cycles more than flight hours are counted for engine maintenance scheduling.

Depends on the engine. Larger gas turbines like those on airliners have maintenance activities dictated by both. My personal experience with them is limited, though, so don’t quote me on that.

Smaller gas turbine engines and piston engines are definitely time-based, not cycle based, maintenance intervals. Oil changes every 50 hours on a Lycoming IO-360, for example, or replacement of the fuel filter on a Pratt & Whitney Canada JT15D-5/5R every… can’t remember exactly how often, but it synched up with a maintenance phase on the Beechjets. Much of it winds up being “on condition”, though.

That said, I distinctly remember that the Honeywell HTF7700 engines on the Citation Longitude have a very short window immediately after shutdown to restart, otherwise they have to wait a certain amount of time (in the 10+ minute range, iirc) before restarting. Something about the shafts sagging/bowing and possibly causing problems.

 

[glappkaeft]

Aircraft engine manufacturers have strong incentive to get this right, as their business model depends on maintenance. One told me that they sometimes even give the engines away for free - they make the money off the contract.

I believe that this is usually called leasing. ;) Which is a common thing when it comes to aircraft engines.

 

[Flyboy]

Especially with airliner engines. Each one can be multiple millions of dollars (or the local equivalent thereof).

 

[Baluncore]

The engines are part of the aircraft. An operator may lease the aircraft with the attached engines. The operator may buy a total engine maintenance package from the engine manufacturer, such as RR now provide.

To increase market share, engines can be supplied at a discount to the aircraft manufacturer. The initial engine cost will be recouped over the life of the aircraft, through the engine maintenance contract.

Regular telemetry of engine performance, to the engine manufacturer, often identifies problems before failure. The reliability of engines is improved when the engine manufacturer provides the monitoring, service, and parts.