Systems question for CRJ pilots

[PCL Flt-ops]

I was studying up on the Garrett APU and was curious about something.

Ever noticed when it's real cold outside and you've got the packs on, occasionally the APU EGT will surge up to around 750C. Then you hear that noise. Is that the LCV modulating the bleed air flow, or is it the altering of the position of the compressor's variable diffusers? I know that both of those are things that will be modulated based on EGT and bleed air.

I know, pretty anal question, but it's been bugging me.

 

[blzr]

You need to spend more time talking to the flight attendant. All that Sh1t is PFM anyway.

(Pure Fu(king Majic)

 

[Socalplt]

LCV modulating and venting off excess pressure. Thats my WAG.

 

[efiscompmon]

I believe both. The LCV does modulate, changing the volume of air through the APU, causing the diffusers to move, lowering the EGT, re-opening the LCV.

Glad I am not the only one (nerd) who noticed it!

 

[PCL Flt-ops]

I know, I am a true nerd!

I think that sound that you hear is the the LCV modulating and bleed air being shed.

 

[Deferals R US]

The CRJ's Apu's VGs only have two positions - high flow and low flow. The VGs are in low flow when you only have one pack running. They automatically move to the high flow position when both packs are on or during an engine start. The sound you hear is the load control valve modulating. If you call maintenance about the APU surging into the yellow they'll just defer it.