"43Inches" Said:
All the turbine aircraft I've flown it gets pretty serious if you exceed the maximum TIT limit at any stage of flight for any length of time.
From one old jet transport manual I have:
Takeoff 915℃ (Redline and 5 minutes max)
Max Continuous 875℃
Max Climb 875℃
Max Cruise 875℃
There's a wide variation among jet engines, depending on probe location, metallurgy, Engine maker, etc. But one thing is common, a high limit for takeoff for some very short period of time (2 to 10 minutes), followed by a series of limits anywhere between 40℃ and 100℃ lower. Some have slightly different numbers for this "series," some are all the same, as above.
They are for slightly different reasons than most of the Limitations in the Recip world. For example, if you operate a recip AT the redline (often 460℉/238℃) the chances of reaching TBO are slim to non-existent, for the engine will "meltdown" first. In the jet engine, operating at redline will cause "stress creep," which will cause the turbine blades to stretch, and the tips to rub against the case, and result in abnormal wear over TIME.
The result in jets is a limit good for some small number of minutes for each takeoff (redline) and a lesser limit for the rest of the time. The promise is that if you obey both limits, your jet engine will last for 20,000 hours or more between overhauls. And they routinely do! There are often further limits on turbine speeds, where they are critical (N1, N2, N3). Modest excursions beyond the REDLINE are handled with a simple inspection (almost invariably returned to service), and larger excursions will trigger replacement of parts. I've never heard of an engine failure due to this, but I suppose there are examples, in extreme cases.
In general, the LIMITS on recips are a little "different." Historically, they (CHT) are set at the maximum structural limits, with strong recommendations to operate well below those limits. Exceptions for the big radials, which have a METO power, and some "flat" engines with real performance (TIO-540-J2DB, for example).
Now, when they started hanging little turbos on "flat" engines, they did not want to install "Turbo RPM" guages, and they wanted "simple." So, they chose a "limit" approximating the "Max Cruise" setting (or less), and came up with "Max TIT." Garrett (most turbos) makes it explicit in their manuals, saying that the "MAX TIT LIMIT" is permissible CONTINUOUSLY for the life of the turbo, AT maximum load (which is max RPM.)
(Note there is no such thing as a "limit" on EGT, as that's more a function of the timing and flame front speed and NOT a measure of "Combustion Temperature.")
The Garrett manuals go on to say, "MAX TIT may be exceeded for purposes of determining peak, or for testing purposes for short periods of time."
Sounds a lot like "Max Cruise" with an unspecified additional margin for 2 to 5 minutes, to me.
I'm quite content to USE the "TIT limit" (usually 1650℉/899℃) for normal cruise, all day long, and to exceed it for determining peak, or setting LOP, etc.
A further note about turbos with Inconel rotors, with limits of 1750℉/955℃. All above also applies to them, BUT airframe makers "abuse the privilege" by installing cheap, lightweight exhaust tubing. It may be "good enough" for the 1650 limit, but NOT NICE for the 1750 limit!
Best...
John Deakin