APU EGT influencing factors?

[daemotron]

I have noticed that APU's EGT does raise when bleed air consumers get connected (engine starter valve most prominently):

However, when hooking up electric consumers to the APU generator (or hooking the whole busses to the APU gen), the EGT doesn't flinch:

I have no real aircraft at hand to verify, but from experience with other aircrafts I would have expected to see the EGT react also on electrical load changes. I couldn't find any videos for the real thing where the EGT display is fully visible on generator hook-up (in most videos it's just covered by the pilot's or FO's hand grabbing the APU GEN connector switches...). But again I don't know how it would work exactly in a 733, so probably someone can enlighten me wethere this is the typical behaviour, of if it's still on IXEG's todo list

[ken_v]

Hi,

Just so happens that I recently made a video of an APU start and generator connect.. however I wasn't specifically looking at the EGT gauge, as these crappy movie stills will probably tell

BEFORE generator connect:

 

AFTER generator connect:

 

Doesn't look like an EGT rise to me.. rather the opposite. Probably EGT still stabilizing after start.

Ken

Edited May 23, 2016 by ken_v

[stefans85]

That's right. The EGT rises when air is consumed. That's why we are adviced to wait 1 min with elec on before activating bleed air especially on cold day's.

[daemotron]

Thanks for clarifying!

[dr_anthony]

You may notice that APU EGT rises then you connect electric consumers on busses. Bleed air consume on eng start get the hard work to APU, and EGT rises rapidly, in hot days it may be nessesary to off some elec consumers prior to eng start.

[DarrenHowie]

Use if bleed air is a far greater load generator on an engine or APU that electrical supply.

same with ground power units which even new big good ones are time limited for starting engines with APU bleed faults.