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Attenuator Warning

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Hi Folks. When flying my Carenado B1009D above 5000' I get a warning saying  'CABIN DIFF HI' Does anybody know what I need to to get it to cancel please?



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I have not flown the B1900 in a while, but to the right of the parking brake, there is a cabin pressure knob that can be turned with the mouse wheel.  It starts at 15000 feet, I believe.. Turn it clockwise to set a higher target altitude and see what happens..


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There was, and may still be, an error in the cabin pressure dial.  [How to deal with this, down below].


I write "may still be" because I haven't yet installed the 20140109 Service Patch.  The notes describing the patch say the pressurization system is improved, but those who have installed it write (in another forum) that it does not correct the problem.  So, maybe if you install that patch it will be corrected, maybe not.  To get the patch you either must have bought the aircraft directly from Carenado, and have an account on their site to get the patch, or (I've read) you can email them some proof of purchase (whatever that may be) and they will give you the download links.


The following information is taken from the "Beech 1900 Airliner Pilot Training Manual" [Rev. 2.01], which can [could] be found on Google.  (If you use this source, you will want to know that the 1900D is series UE.)


Okay, so the problem is that the cabin pressure dial is not labelled correctly -- specifically the inner ring of numbers is incorrect.  Unfortunately, those are the numbers which are supposed to correspond to the cruising altitude.  (If we just had a couple of corrected gauge image files, the system would work as it should.)


To be absolutely correct, on the pressurization dial, the aircraft altitude is supposed to be set for 1000 feet above the actual cruising altitude, so if you fly at the service ceiling of FL250, the aircraft altitude on the cabin pressure dial should be set to 26[000].

(In the real world, it's actually more complicated than this for delayed / step climbs and airfields at different elevations, etc., this information also in the Pilot Training Manual.)


The following is a list of cruise altitudes, the aircraft altitude ["ACFT"] which should be commanded (on the pressurization dial) [if the dial was correct], and the cabin altitude on the dial which can be used.


10000 and below --> 11000 --> set to 0 and you may want to open the "Dump" switch.


Cruise Alt --> +1000  --> Set Cabin Altitude


11000 --> 12000 -->   600 feet

12000 --> 13000 --> 1300 feet

13000 --> 14000 --> 2000 feet

14000 --> 15000 --> 2700 feet

15000 --> 16000 --> 3300 feet

16000 --> 17000 --> 4100 feet

17000 --> 18000 --> 4800 feet

18000 --> 19000 --> 5400 feet

19000 --> 20000 --> 6100 feet

20000 --> 21000 --> 6700 feet

21000 --> 22000 --> 7250 feet

22000 --> 23000 --> 8000 feet

23000 --> 24000 --> 8550 feet

24000 --> 25000 --> 9200 feet

25000 --> 26000 --> 9800 feet


The corrected gauge would look something like this.  If I made a mistake in the text table above, this image is the correct one of the two.



Compare this to the dial in your cockpit and you'll see that if you plan to cruise at FL210 (adding 1000 feet = 22000), your dial would have you set the cabin pressure to just under 4000 feet.  But it really needs to be set to about 7250.  So, your dial commands a pressure which is 3200 feet different than is appropriate.  Therefore, you get the pressure differential warning.

(If you think about it, this is why it seems possible to turn the dial (in the Carenado B1900) to altitudes much higher than the service ceiling 25000 feet.  It looks like it can be set to an aircraft altitude of something like 33000 feet.  But, if the dial were corrected, you would see that it only can be turned to 26000 for a 25000 foot actual cruise.)


One more thing, about using this pressurization system.


Even with the proper setting you can still run into a problem (this is not a bug).  The 1900D can climb much faster than default cabin climb rate [set with the rate knob].  If the cabin rate increases too slowly compared to the actual rate of climb, you will still get the warning . . . for a while, once the cabin catches up, the annunciator will clear.  If you are trying to fly "realistically," this should be avoided.


Making things a little more tricky, the cabin climb rate gauge will not register until the wheels leave the ground, so it's not even possible to know what rate you've set until you're in flight.  With all the take-off details and speed, this can be forgotten.  This is the reason why checking the system during climb-out is (or should be) part of the checklists. It is necessary to compare the cabin [pressure] climb rate against the actual climb rate, watch the pressure differential gauge, and adjust the rate knob as needed.  (I find turning it clockwise to about the 5 or 6'oclock position during pre-flight is usually about right.)


Same attention is needed while descending, set the altitude to 1000 feet above the destination field and adjust the cabin rate per the actual aircraft descent rate.


Good luck and good flights.

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