Couple APU questions.

J

JordanD

Here so I don’t get fined
Being out on the ramp has made me pretty curious about some of the planes I've been working around. What causes the ERJ APU to sound like it's spooling up and down? I only notice it on a few aircraft (seems like the same few in particular that do it.) Also, what causes the CRJ (only notice it on the -100/200s) APUs to randomly make a loud PSSSSSSSSHHH noise while they're running? I swear that thing is an instrument of the Devil.
 
It's the Load Control Valve (LCV) opening and closing to regulate the airflow when the air conditioning PACKs are on. When you're in the cockpit on the CRJ, you can watch the temperature spike up 100 degrees or so when the APU makes the loud noise.
 
On the ERJ, the loud, debilitating whistle you hear is coming from either of the wing roots, where the ECU's (Environmental Control Units) are located. The surge comes from the tact-lacking "Set down switch", which tells the PRSOV (Pressure Regulating and Shutoff Valve) to open and close. Basically, that is the very annoying regulation of the temperature in the airplane. The APU itself isn't very loud unless you're back by the tail. If it weren't for those stupid ECU's, I'd be able to do a walkaround and just cover my ears at the tail.
 
I always ran both packs in the Barbie fun jet at the gate.

I know it was soo taboo, but it seemed to kill the cycling noise, and kept the cockpit comfortable. I always got lots of complements from the FAs how nice the temp always was.
 
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