I’ve always wondered why when departing an airport ex: 4000’ asl the cabin pressure descends after takeoff significantly I.e. 1500 feet approx the others day. Is the pressurization system descending to follow a schedule? This happens on a challenger, citation and hawker so it is intentional.
Can anyone explain?
Why doesn’t the cabin just stay at t/o alt until the schedule catches up?
Cabin pressure descend on takeoff from higher alt airports
Moderators: lilfssister, North Shore, sky's the limit, sepia, Sulako, I WAS Birddog
Re: Cabin pressure descend on takeoff from higher alt airports
How does the controller work on your planes: do you input cruising level and/or destination airport elevation?
As an AvCanada discussion grows longer:
-the probability of 'entitlement' being mentioned, approaches 1
-one will be accused of using bad airmanship
-the probability of 'entitlement' being mentioned, approaches 1
-one will be accused of using bad airmanship
Re: Cabin pressure descend on takeoff from higher alt airports
Alt sel is set to cleared altitude and FMS does have cruise alt pre set and destination airport alt.
Re: Cabin pressure descend on takeoff from higher alt airports
i think the planes can’t differentiate between “just took off” and “been climbing for half an hour”. if, during a climb from sea level, you lose a bit of pressurization, the controller will increase pressure to get back on a normal climb schedule. if you just take off, the controller might assume, for whatever reason, that you lost some pressurization and is attemptin to compensate.
what you are looking for would be a feature that adds a different climb or descend mode in your controller.
what you are looking for would be a feature that adds a different climb or descend mode in your controller.
As an AvCanada discussion grows longer:
-the probability of 'entitlement' being mentioned, approaches 1
-one will be accused of using bad airmanship
-the probability of 'entitlement' being mentioned, approaches 1
-one will be accused of using bad airmanship
Re: Cabin pressure descend on takeoff from higher alt airports
I am not exactly familiar with types listed.
But it may be that the cabin altitude for that flight is below the pressure altitude for takeoff. Usually the outflow valves are open by default on the ground and will begin to close in the takeoff roll, or tied to a squat switch that may be open until liftoff. Following which it must descend to reach the cruise cabin altitude.
It could also be the fact that most pressurization controllers especially on older aircraft are pretty “dumb”. So it may target and try to maintain a set psi differential and rate of change of psi as you climb. And when I say it “tries to maintain” I don’t mean to imply it is thinking. A lot of older aircraft just have a vacuum line between the controller and outflow valve, and it just works like a pneumatic relay. Pretty Mickey Mouse.
But it may be that the cabin altitude for that flight is below the pressure altitude for takeoff. Usually the outflow valves are open by default on the ground and will begin to close in the takeoff roll, or tied to a squat switch that may be open until liftoff. Following which it must descend to reach the cruise cabin altitude.
It could also be the fact that most pressurization controllers especially on older aircraft are pretty “dumb”. So it may target and try to maintain a set psi differential and rate of change of psi as you climb. And when I say it “tries to maintain” I don’t mean to imply it is thinking. A lot of older aircraft just have a vacuum line between the controller and outflow valve, and it just works like a pneumatic relay. Pretty Mickey Mouse.
Re: Cabin pressure descend on takeoff from higher alt airports
Bang on. Hawkers, you open the bleeds manually after takeoff to start pressurization which always causes a bump.
Re: Cabin pressure descend on takeoff from higher alt airports
Got the answer from bomrdier....all the planes I fly are new in the past three years.
Bom said the fms follows a predefined cabin climb schedule. When taking off from a higher alt airport after takeoff it descends to meet that schedule then climbs normally.
Bom said the fms follows a predefined cabin climb schedule. When taking off from a higher alt airport after takeoff it descends to meet that schedule then climbs normally.
