B763 APU question

[The Dominican]

A question for the gurus out there. I have noticed that the EGT indication at cruise never falls under single digits, I have seen it at 6c to 9c and even on long flights it never falls bellow this numbers. The APU casing is not part of the pressure vessel so it is exposed to very cold temperatures that orbit around the -50c for hours on end, I would imagine that there is radiation heating associated with hydraulic lines, electrical equipments, the dome of the pressure vessel, etc. that would raise the temperature somewhat in the tail cone section but even if the temperature is 20c warmer due to those factors (I doubt it would be that much) it would still be -30c??? so I can't help to think that the APU casing is heated somehow? Or is it that the EGT temperature sensors don't go bellow this indications(although it does get colder than that)?

Thank you for your time

[Clipped]

1. The APU compartment is NOT heated.

2. My guess is that it is the nature of the reaction of the Chromel-Alumel thermocouples to heat that provides an EGT signal to the ECU, as such, you do not get a 'reading' of cold.

[Short_Circuit]

I suggest the cold junction is in an unpressurised area.

[prattgamco]

EGT PROBES ARE IN APU EXHAUST DUCT.SO THEY ARE NOT CONCERNED ABOUT AMBIENT TEMP R APU COMPARTMENT TEMP.THEY SO THE TEMP EXISTING AT THE APU TURBINE OUTLET.WHEN THERE IS NO AIRFLOW(-56C IN CRUISE) THROUGH THE APU COMPRESSOR AND TURBINE I GUESS THEY SENSE ONLY APU EXHAUST DUCT TEMP.NEED MORE HELP TO SHORT IT OUT:ugh:

[Short_Circuit]

EGT PROBES ARE IN APU EXHAUST DUCT.SO THEY ARE NOT CONCERNED ABOUT AMBIENT TEMP

But, the exhaust duct is open to ambient @ -56C also...
EGT is temp above ambient.... not above cabin temp. So the cold junction must be outside pressurised area or electronically compensated for ambient. T.C's work because of a difference in temp between the hot junction (in the turbine) and the cold junction (could be anywhere) a voltage is generated proportional to the difference and displayed in temp. So when both hot & cold junction are at the same or similar temp, there will be only fractions of a microvolt being generated. Induction from other A/C systems may now affect the circuit and give false readings. :8

[ampclamp]

thermocouple outputs are non linear and are designed to work accurately in a given range.
apu egt is not an OAT gauge and is designed for accuracy in temps say 200 to 900 c.
Much less and its not turning and burning, hence not really relevant.
My guess is that may explain the observation you have made.

[The Dominican]

Thank You Gents

[Short_Circuit]

Then there is the possibly that the cold junction is in the APUC which is in the aft cargo sidewall and everything I said is then crap! :eek:

Would need to see the SSM to be sure. :hmm:

[Fargoo]

APU controller doesn't monitor the output of the thermocouples when the APU has shutdown.
If you ever watch a 767 or 757 shutdown on the PERF/APU page or Status page of EICAS you'll notice the EGT drops from 200+ to about 8 degrees almost instantly when the RPM drops to single figures on shutdown.

It then remains at 8 or so degrees regardless of whether the aircraft is in Dubai or the depths of Alaska until the APU is switched to on again.

No fancy thermodynamics at work just an EICAS that reads 8 or so when it has no signal from the APU controller.

Hope this helps :ok:

[Fargoo]

Also FWIW the output of the thermocouples has to be between -100 degrees F and +2000 degrees F for the APU controller to consider their output as valid.

Quite a wide temp range :ok:

[Swedish Steve]

So we need someone to turn on the APU controller in the cruise, and see what the EGT reads.

There are other discrepancies at low temps. Here in ARN we have winter morning temps of M10 to M15 degC.
I had two nightstoppers last winter, an A320 and a B757. One of them (can't remember which way round!) had engine oil temps and EGT showing M10 in the morning, the other one was always Zero. It wasn't programmed below zero.