EGT limit on start

[Private jet]

A collegue of mine asked me the other day why the EGT was limited on start to a lower temp than max or max continuous. As an ex a/c eng i said it was due to the fact that the internal air cooling of the turbine blades was not up and running at start up, hence the lower temp. Was this the right reply?

[BelArgUSA]

The right reply is this one -
xxx
All engines different types have different EGT rise during start.
First of all, where are the Tt probes - are they Tt5, Tt7, Tt9 ???
xxx
I have seen jet engines staying very cool and slow to get EGT rise...
The J-85 CJ-610 / Learjet 20s... looks like a hot start on every start.
EGT exceeds well above red line during start... by 150/200 degrees...
xxx
:ok:
Happy contrails

[N1 Vibes]

Private Jet,

yes - your colleague was correct. Becaue the engine is not running at idle or above the cooling air to the turbine mahinery (discs, blades,shafts etc) is not normally sufficient to cool an EGT overtemp of the normal running range.

Regards,

N1 Vibes

[lomapaseo]

There is a well behave relationship, with the engine running, between EGT (Exhaust Gas Temperature) at the very back of the engine and TIT (Turbine Inlet Temperature) at the highest temp location at the front of the turbine. This relationship can not be depended when starting an engine if the combustion is not steadied in the burner with increasing burner pressure. Abnormaly high EGT during start signals that all is not right and its time to start over.

[BelArgUSA]

Are you so sure about that...?
I shall give you some figures for the CJ610-6 (Learjet 24B).
xxx
EGT temperature markings are "green arc" goes up to 702ºC
The "yellow arc" - 5 minutes operation (T/O) is 702ºC to 716ºC
EGT "red line" - 716ºC - 5 minutes T/O limitation.
xxx
There is a second "red line" which is a momentary start limitation - at 950ºC
It is common to see EGT rising (fast) during start, generally to some 800ºC
FSI recommends to call it "hot start" if EGT continues rise beyond 850ºC
That is - within 100ºC of start "second red line".
Looks like hot start to all civilized pilots, except if familiar with these engines.
xxx
According to my friends engine maintenance technicians -
Excessive start EGT is often due to HP bleed ports closing malfunctions.
That applies to the JT3D, JT8D and JT9D, engines I am most familiar with.
xxx
A certain JT9D type might have a 950ºC EGT red line (Tt7).
A hot start is assumed if EGT rises rapidly above 500ºC.
For these type engines, a high FF is still the first clue of impending hot start.
Poor N1 or N2 acceleration (low bleed air start pressure) is the other clue.
xxx
As usual - these are my classroom/simulator briefings.
If they are not as per "your SOPs" - just "fry your engines" as you wish.
Melted turbine blades are delicious, with a little Tabasco and salt.
xxx
:}
Happy contrails

[SNS3Guppy]

Start temperatures are provided for a number of reasons, but with one basic underlying intent; prevent engine damage.

Temperature as reported in the cockpit may be sampled at a number of different locations, depending on the type of engine which you're operating. Exhaust Gas Temperature, Interstage Turbine Temperature, etc, are different ways of describing the same thing; the temperature at some point in the exhaust flow. The problem is that during an engine start, the temperatures at other parts of the engine may be substantially different...you may have a low temperature exhibited at the tailpipe or between turbine stages, whereas a very high temperature may be occuring inside the burner can or the turbine inlet. Without a direct means of identifying what that temperature is, then limiting the start temp to a lower value is one way of ensuring that components aren't damaged during the engine start.

A hung start is a good example. During a hung start, inadequate airflow is occuring through the engine. The flame isn't going where it's supposed to, and you can't be assured of adequate airflow protection through the burner can to keep the flame off the burner can walls...you may also be experiencing streaking or flame propogation beyond the can...which can cause damage to turbine inlet guide vanes or turbine blades downstream. The EGT, ITT, or TiT, by be indicating a low temperature, but you may have a very hot, raging fire going on upstream of the temperature probes. You may be experiencing engine damage and not even show a temperature which appears to be harmful.

The same can be true of a normal start. Depending on where the temperature probes are located, you may have already experienced catastrophic engine damage before the temperature ever reaches a point on the EGT or ITT or TiT indicator that might appear harmful.

Airflow through the engine, and the conduct of heat and gasses, isn't the same during the engine start sequence as it is when the engine is running in a stable fashion.

Many engines use multiple stage nozzles which open or increase fuel flow at various points in the start, in order to bring the fuel flow up in stages as increasing airflow is available in the burner can to help keep the flame off the burner can walls. This is a very crucial time for the engine, as excess fuel will create temperatures that won't necessarily register as high on the cockpit indications, but could still easily cook internal components. As no cockpit indication exists to warn of this damage beyond a low limited start temperature, your first indication might just be shedding a turbine blade or an uncontained fire outside the burner can. As a result, some engines (due to the placement of the thermocouples in the temperature indicating system) do have a start temp which is lower than the other operating temperatures.

You may also be thinking of start temperatures which are used for cutoff purposes. The actual start limit may be 1100 degrees, for example, but the engine may have a start cutoff limit of 600 degrees. The reason for this is that normally to prevent damage you must cut off fuel if the temperature approaches the 600 degree point...but the damage limit is considerably higher. You can cut off the fuel early, save the engine, and perform another start...but if the temp hits the start limit redline, then you've got a hot section inspection and teardown coming. The specifics for the relationships of these numbers should be spelled out exactly, or in general, in the maintenance publications for the powerplant in question.

[V1... Ooops]

On certain types of engines (the PT6A family comes to mind), the starting temperature limits are actually significantly higher than the takeoff or MCP limits.

As others have already mentioned in replies above, there are a great number of variables that will affect what the indicated temperature limits during different phases of engine operation are.

One, of course, is where exactly the temperature sensors are located. Typically, they won't be located at the hottest part of the engine, therefore there is a relationship between measured temperature and the critical temperature at another point. This relationship changes depending on the nature of the engine operation - start, idle, or high power.

Next is the lag inherent in the measuring system. The measuring system is usually optimized for high power settings, and therefore might not react quickly enough to catch peaks during starting - thus resulting in a reduction in the indicated temperature limit during a start cycle.

Last, on certain types of engines, much higher temperatures can be tolerated during a start because the engine parts are not yet heat-soaked, and the rotation speeds are low. By way of example, you can pass your hand through top of the flame from a candle and not burn yourself - providing you don't leave your hand there long enough to let it become heat-soaked.

I think it would be very difficult to provide an answer to your question that is valid for all engine types.