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Pressurization doesn't work correct

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Most of my flights with the C90B are managed at about FL200. So there is no problem to set ACFT ALT in Pressurization Controller at 20 and everything works fine. First bug (but allready known): pressurization works also with Bleede Air Valves closed.


Then I was wondering whats about the HI CAB ALT alert. I had this alert twice when I set the CAB ALT in Pressurization Controller to a bit more than 10.000 ft. Okay, it was easy to set altitude a bit less and the alert was stoped.


But what happens when I climb up to FL300? As I have understand the Pilot Training Manual, the max. pressure difference between inside and outside is 5 psi. With max. CAB ALT 10.000 ft this difference will be reached at about 26.000 ft. Climbing higher:


Pilot Training Manual:


(...) If the airplane climbs

to an altitude higher than the value indexed on

the ACFT ALT scale of the dial on the face of the

controller, the pressure differential will reach the

pressure relief setting of the outflow valve and

safety valve. Either or both valves will then override

the cabin pressurization controller in order to

limit the pressure differential to the maximum

pressure differential. If the cabin pressure altitude

should reach a value of 10,000 feet (12,500

for LJ 1353 and later), a pressure-sensing switch

will close. This causes the red ALTITUDE

WARN annunciator light to illuminate...


BUT here is the second bug: Cabine Altitude is not increasing, when climbing up to FL300, no red ALTITUDE WARN annunciator light is visible, but the pressure differential is growing over the maximum of 5 psi into the red arc, which is not right and impossible

because of the savety and outflow valves.



FL300: Cabin Altitude 10.000, DIFF PRESS 5,8 psi, no HI CAB ALT annunciator light



I think this is another point in our to-do-list for SP3.

Allways three greens,


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The pressure differential is between you and the outside air. Check first whether your cabin altitude should be set as high as 10,000ft. Check also your operational ceiling. and check your manuel for the max pressure differential allowed.

Once your pressurisation system has arrived at the cabin altitude it won't change unless there's a leak or a relief valve opens to preserve the differential pressure. If your differential pressure should be no more than 5 then obviously a relief valve hasn't opened.

Or possibly the gauge programming doesn't model everything in full. On my VC10 update I have 12 visible elements and some 30 background elements just to control the pressurisation. Add to that another 26 elements for the oxygen delivery system of which only 9 are visible to the pilot, you can see it is quite a complex model.



Super VC10 into LOWI with PF3 at a cinema near you



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The pressurization system in the real King Air is not complex.


The OP is correct in stating it's not working correctly.

Ed Wilson

Mindstar Aviation
My Playland - I69

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