EEC HARD ALTERNATE/OVERSPEED/overtemp

[de facto]

Hello,

Does your Airline provide you with info on how to avoid low pressure compressor overspeed and or EGT excedance while in the EEC Hard Alternate mode?
The QRH is quite vague on the failure itself and does not provide crews with a way to avoid such an exceedance.

Would flying at lower Mach Number if at the optimum level by reducing the cost index and therefore your Mac cruise speed avoid the low compressor overspeed a satisfactory method?
Would the use of the manual n1 setting in cruise allow us to avoid EEC calculated N1 overshoot ?
For the Go around,would the n1 limit in the QRH be sufficient to avoid a compressor and or EGT exceedance?

Why does the 737 have an Overspeed governor for the turbine but not for the compressor?

Thanks.

[172_driver]

The FCOM2 states that N1 and N2 overspeed protection is provided both in normal and alternate mode.

As far as I understand you can still overboost (too high internal pressures) in alternate mode. In soft alternate engine parameters (e.g. limiting N1) are defined from the last valid environmental conditions. So if the air would get any colder/more dense you could potentially cause damage if you would set max N1.

[de facto]

Hello 172driver,

The FCOM2 states that N1 and N2 overspeed protection is provided both in normal and alternate mode.

In normal and SOFT alternate mode ,that would be correct.
However in HARD alternate mode,there is no engine compressor speed protection anymore,only engine turbine protection via the overspeed governor.
My question is in case of HARD Alternate mode,what would be possible actions to avoid a compressor overspeed/EGT over temp in hot days conditions.



Thanks for your input.

[172_driver]

Ok, thanks for that. Learnt something new

[Swedish Steve]

Why does the 737 have an Overspeed governor for the turbine but not for the compressor?

Because the compressor and turbine are bolted together.

[de facto]

Now this one of the reasons i had,the low pressure turbine is turning the low pressure compressor.Therefore a governor reducing fuel and speed of N2 will reduce N1.
I am looking for practical soulutions in flight to avoid overspeed of compressor with A/t engaged.
Thanks swede.

[lomapaseo]

I don't understand the basis for the question ... it could be just me

Why can't you assume that the engine controls FADEC etc. will protect all the spools from overspeeding beyond the engine manufacturers limits for all operating modes ?

After all it is the manufacturer who defines and certifies the engine to such limits

[de facto]

Why can't you assume that the engine controls FADEC etc. will protect all the spools from overspeeding beyond the engine manufacturers limits for all operating modes ?

Because it is simply not the case.
Following a reversion to soft alternate,the qrh leads pilot to switch the eec into the hard alternate mode.

[mono]

I'm not aware of any FADEC that prevents an EGT excedence whatever mode it's in. If you're in trouble you want full thrust even it it might result in an EGT overtemp.

As for turbine overspeed. I suggest you control the engine and watch the speeds just like they used to.

[lomapaseo]

As for turbine overspeed. I suggest you control the engine and watch the speeds just like they used to.

Why??

For EGT I accept that deterioration thrust outside temp and altitude may tke you over limits leading to degradation and shortened life over time.

But rpm has mechanical limits that are protected against via FADECs etc. in todays engines even if you firewall it.

[de facto]

But rpm has mechanical limits that are protected against via FADECs etc. in todays engines even if you firewall

Not in hard alt mode...n1 rpm not protected..only N2.

mono
*
Join Date: Feb 2002
Location: The Sandpit
Posts: 490
I'm not aware of any FADEC that prevents an EGT excedence whatever mode it's in. If you're in trouble you want full thrust even it it might result in an EGT overtemp.

As for turbine overspeed. I suggest you control the engine and watch the speeds just like they used to.

Correct,the EEC does not protect for EGT exceedance unless during engine start,it will shutoff the fuel after 15sec if oat above 2c and 20secs if temp below.