Thermal Shock
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Disgusted of Tunbridge
Disgusted of Tunbridge
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That's not much thermal shock. Nowhere nearly as much as, for example, a go around where the engine goes from not much above idle power to go-around power, or an immediate shutdown with an unrecoverable surge
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On the 757 there is about 30deg C increase in EGT when clb1 is selected at high altitude. Obviously there are flight phases like a go-around or take off that has much larger increases in EGT and yet it is quite widely taught in my company that it is big no-no to select clb1 in the last 2000ft of climb because of the possibility of 'thermal shock'.
I have never seen this written down and suspect it maybe an SOP left over from the dark ages.
Any comments?
I have never seen this written down and suspect it maybe an SOP left over from the dark ages.
Any comments?
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Having flown older jet transports from the 'dark ages', I would suggest that your management is out to lunch, with regard to any 'thermal' shock' in the later stages of the climb....IE: never heard of it.
Jet engines are designed and certified to be safely used throughout the flight regime without restrictions. Tis true that they do wear by virtue of ablation, rubbing and cyclic fatigue, but this is to be accounted for by maintenance schedules.
So perhaps your operational department wants to baby the engines to save some bucks by extending maintenance schedules
So perhaps your operational department wants to baby the engines to save some bucks by extending maintenance schedules
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In the dark ages, some pilots would regularly flash thei landing lights ON when an approaching aircraft was seen.
Some person in a position of authority decided that the lights would be subjected to 'Thermal Shock' if switched on at -45C.
It entered the Ops manual and became SOPs not to do such a thing.
When it was pointed out to him that the filament reaches a temp of 3000C (three thousand) then the starting temp variation from +25 to -45C seems particularly insignificant.
The same can be said of modern Jet engines.
It's a myth.
btw, deleting CLB 1 or CLB 2 will reduce the fuel consumption so very soon you will all be doing it when selecting UP speed or flap retraction.
Bean counters rule.
Some person in a position of authority decided that the lights would be subjected to 'Thermal Shock' if switched on at -45C.
It entered the Ops manual and became SOPs not to do such a thing.
When it was pointed out to him that the filament reaches a temp of 3000C (three thousand) then the starting temp variation from +25 to -45C seems particularly insignificant.
The same can be said of modern Jet engines.
It's a myth.
btw, deleting CLB 1 or CLB 2 will reduce the fuel consumption so very soon you will all be doing it when selecting UP speed or flap retraction.
Bean counters rule.
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A turbine engine experiences a lot more "shock" during the engine start than any transitions in flight, be it cruise to idle on a descent, a missed approach power increase, or otherwise. In fact, it's harder on a turbine engine to shut it down without a normalization period, than to make power transitions.
As a mechanic (engineer, to some), I've handled burner cans that looked like shattered swiss cheese. You might be surprised at the amount of damage that's normal in a turbine engine burner can. Damage such as cutting and burning on turbine inlet guide vanes, blade creep, rubbing, etc, are all bigger issues on rotational parts. Turbine engine parts are air cooled, even with an influx of fuel (with a power increase), a proportionately greater influx of cooling airflow takes place. Even in the burner can, where a layer of air normally prevents flame contact with the can walls.
The single greatest thermal shock to an engine occurs during engine start, of course...but this isn't a shock. The only thermal damage we're concerned with is overtemperature...which can damage blade coatings and blades, damage the metalurgy, cause streaking (raw fuel downstream from the nozzles, which causes spot burns right through the cans or burns on the inlet guide vanes or turbine blades...or torching with nozzle damage), etc. Thermal damage can occur during a compressor stall in which the temperature may rise substantially higher in the can (and even reverse flow to some degree, as expanding, burning gasses tend to stop moving aft and expand in both directions) or sections forward of the can...but not immediately register on EGT or ITT gauges (because the fire is going on somewhere other than the temperature probes). That's also part of the reason for start limits which are lower than takeoff limits, incidentally; it's two-fold. The internal temps aren't turbonormalized yet...so the limits are lower. Additionally, the temps going on inside the engine may be higher initially than what you're seeing on the gauges until the engine is stabilized and on-speed...it gives you margin. In any case, that's where your greatest thermal threat lies.
So far as pushing the power up manually and "shocking" the engine, don't forget, it's self heating, and it's continuously normalizing itself (preheating,if you will), so it's ready when you push the power in.
A turbine engine is full of dissimiliar metal; it expands at inconsistant rates, and it cools at inconsistant rates. During normal operation everything is preheated and kept that way, all the parts function as a team. After shutdown, if the engine isn't given a brief period to normalize (or to get fairly uniform temperatures throughout), then certainly the rates at which some parts continue to increase in temperature (some do, after shutdown, believe it or not) and others begin to rapidly cool can cause siezure or rubbing or other problems...part of the reason that many manufacturers provide a minimum idle run time prior to shutdown (often two or four minutes).
Anyone who's ever run a Garrett TPE-331 and had to spin the blades by hand after landing, or who's experienced shaft bow due to the temperature differentials gets the idea.
As a mechanic (engineer, to some), I've handled burner cans that looked like shattered swiss cheese. You might be surprised at the amount of damage that's normal in a turbine engine burner can. Damage such as cutting and burning on turbine inlet guide vanes, blade creep, rubbing, etc, are all bigger issues on rotational parts. Turbine engine parts are air cooled, even with an influx of fuel (with a power increase), a proportionately greater influx of cooling airflow takes place. Even in the burner can, where a layer of air normally prevents flame contact with the can walls.
The single greatest thermal shock to an engine occurs during engine start, of course...but this isn't a shock. The only thermal damage we're concerned with is overtemperature...which can damage blade coatings and blades, damage the metalurgy, cause streaking (raw fuel downstream from the nozzles, which causes spot burns right through the cans or burns on the inlet guide vanes or turbine blades...or torching with nozzle damage), etc. Thermal damage can occur during a compressor stall in which the temperature may rise substantially higher in the can (and even reverse flow to some degree, as expanding, burning gasses tend to stop moving aft and expand in both directions) or sections forward of the can...but not immediately register on EGT or ITT gauges (because the fire is going on somewhere other than the temperature probes). That's also part of the reason for start limits which are lower than takeoff limits, incidentally; it's two-fold. The internal temps aren't turbonormalized yet...so the limits are lower. Additionally, the temps going on inside the engine may be higher initially than what you're seeing on the gauges until the engine is stabilized and on-speed...it gives you margin. In any case, that's where your greatest thermal threat lies.
So far as pushing the power up manually and "shocking" the engine, don't forget, it's self heating, and it's continuously normalizing itself (preheating,if you will), so it's ready when you push the power in.
A turbine engine is full of dissimiliar metal; it expands at inconsistant rates, and it cools at inconsistant rates. During normal operation everything is preheated and kept that way, all the parts function as a team. After shutdown, if the engine isn't given a brief period to normalize (or to get fairly uniform temperatures throughout), then certainly the rates at which some parts continue to increase in temperature (some do, after shutdown, believe it or not) and others begin to rapidly cool can cause siezure or rubbing or other problems...part of the reason that many manufacturers provide a minimum idle run time prior to shutdown (often two or four minutes).
Anyone who's ever run a Garrett TPE-331 and had to spin the blades by hand after landing, or who's experienced shaft bow due to the temperature differentials gets the idea.
What springs to mind is the "core lock" that CRJ suffered from a couple years ago. As far as I remember, they went to a level above normal operating - an altitude ok in ISA, but under actual conditions above service ceiling-, engines stalled and due to low speed and low OAT the hot section was not that hot any more. tried restarting to no avail and crashed later on.
Or something to that extend..
Or something to that extend..
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Understand, Ray...but 'core lock' is a totally different subject.
And yes, it could indeed happen with older jet engines....or, as the happless CRJ crew found out...with newer designs.
The silly fools.
And yes, it could indeed happen with older jet engines....or, as the happless CRJ crew found out...with newer designs.
The silly fools.
