News: Added Links For Twin Commander and Facebook Pages

Login  |  Register

Author Topic: The Fairchild Environmental System  (Read 483 times)

Mark S

  • Newbie
  • *
  • Posts: 17
The Fairchild Environmental System
« on: July 07, 2021, 05:18:02 pm »
Not much discussion about this particular unit.
The 690B Fairchild unit is a heavy duty reliable 75 lb. monster of a machine.  Seems reliable.
I do get some unusual fluctuations on the pressure system though.
Occasionally, but not always, the cabin pressure will change when I select a different cabin temp. in the AUTO mode.  Sometimes the pressure change is small, other times its an 800 or 900 FPM change.

Very sloooww change in temp. selection helps.
I wonder if this in common to the type, or I should have it looked at?


  • Administrator
  • Hero Member
  • *****
  • Posts: 2555
Re: The Fairchild Environmental System
« Reply #1 on: July 08, 2021, 03:00:39 pm »
That is all pretty common stuff.

Commanders (well, other than the 900/1000) tend to have fairly leaky cabins, so the airflow in matters a lot. The more heat you ask for, the more air flow you will get, which can result in a decrease in cabin altitude. The opposite is true for asking for more cold air-- you will get less air flow.

It may be a sign that your cabin is starting to get a bit leakier than normal, not a big deal-- often it's the rudder pedal boots which are at fault, although any good Twin Commander shop or service center should have no issues trouble shooting and improving it. But to some extent, it is just how it is, even with the tightest cabin.

Bruce Byerly

  • Hero Member
  • *****
  • Posts: 746
Re: The Fairchild Environmental System
« Reply #2 on: July 09, 2021, 12:51:42 am »
The cabin should not fluctuate in auto if itís tight. If itís marginal, auto will lead to fluctuations so manual mode (override) will stop the fluctuations auto brings every time the system calls for additional bleed air aka heat. Itís a battle but maintaining the cabin so itís tight is the best approach imho.