# Why does the 737NG EGT drop during take-off?

I noticed that the EGT drops a bit when I hit TO/GA for take-off on a 737-800.

• I think you may add some context (it may depend on ambient pressure and temperature) Feb 27, 2020 at 6:52
• @ManuH Seems like a perfectly valid question to me. Feb 27, 2020 at 7:31
• @J.hougaard I agree. Adding context will bring this question from valid to well documented. Feb 27, 2020 at 9:57
• Is this observed in the aircraft, the simulator, or in a program on a computer? Feb 27, 2020 at 10:13

Above is takeoff engine data$$^1$$ and is indicative of the behavior you've mentioned. (FF is fuel flow and TLA is thrust lever angle.) The same behavior can be seen here as well:

(The exact timing may depend on how long the engines are left in the stabilization process.)

Note: The 737NG EGT sensors are at the T495 station in the low pressure turbine section. There are 9 thermocouple probes and their average is displayed.

The CFM56 employs variable stator vanes (VSV) in the compressor. In general those vanes close at low engine speed and open at high engine speed (shown below).

Source: CFM56-5A manual

The specifics are proprietary, but plausible conclusions can be made. When the engines are stabilized for TO/GA, it could be the ECU (electronic control unit) opening the VSV to provide the ideal compressed air for faster engine acceleration (more air before the fuel would lower the EGT).

Note: The VSV is controlled by the hydromechanical unit (HMU), which receives input (torque motor current) from the ECU (CFM presentation, PDF page 16).

Of course if you have cause for concern, don't use the internet for flight, and check with your airline.

$$^1$$ Lion Air Max crash report (sadly the only public source of FDR data I can think of is accident reports).

• wow sir thank you, i am not an engineer by background any may need some time to process this. but it's quite interesting to know why! Apr 29, 2020 at 12:48