# Pressurization Time

#### montanapilot

##### Well-Known Member
The post in the general topics got me thinking.

In a pressurized plane how quickly does the presurization react to changes in altitude?

Say you were pressureized at an extremely high altitude and dived 7000 fpm, would the pressurization system be able to react quickly enough to keep everyone comfortable or not.

#### Tired

##### New Member
Assuming you set the rate controller correctly, yes it would. At 35,000ft a typical cabin alt is 6-7000ft, so even if you dived at 7000fpm you'd only have to loose around 1200-1400fpm of cabin alt to keep up.

#### FalconCapt

##### New Member
Yes, the pressurization can typically keep up, that is until you descend to the cabin altitude, then the cabin will descend at the aircraft rate.

One late night in the Falcon 50EX we were empty and did a rapid descent (FL430 to 10,000 ft) in just under 2 mins. (about 18,000 FPM) When we started, the cabin altitude was about 7,300 ft, when we got to 10,000 ft the cabin was about 6,400 ft, as far as the pressurization, it responded normally. If we would have went below about 6,000 ft the cabin would have started to descend at the airplane rate, which would have been very hard on the ears.

Hope this helps...

#### say_speed

##### New Member
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One late night in the Falcon 50EX we were empty and did a rapid descent (FL430 to 10,000 ft) in just under 2 mins. (about 18,000 FPM)

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Whoa, 18,000 ft/min, what made you do that? I didn't even know the F50 could achieve such a rate of descent!

#### FalconCapt

##### New Member
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Whoa, 18,000 ft/min, what made you do that? I didn't even know the F50 could achieve such a rate of descent!

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Boredom and being empty in the middle of the night!

We let the speed bleed back at FL430 to M0.70 before starting the descent, then at idle power, extended speed brakes to full and descended at M0.86 until transitioning to 370 KIAS and maintained that speed throughout the descent. It came down quite nice, the controller said he hadn't seen that kind of descent rate except for the F-15's and F-16's doing rapid descents. We did inform the controller before doing this descent that it would be very rapid, he said there was no one below us all the way to the ground.

#### FL270

##### New Member
When flying corporate, many of us take the time to practice things that we usually only do in the simulator -- but can be SAFELY done in the actual aircraft -- when we happen to have an empty leg. On my empty legs I've done emergency descents, no-flap landings, maximum effort landings, go-arounds, steep turns, etc. It's good practice and adds some additional value to a routine ferry flight.

In fact, I need to do a go-around on my empty leg in the morning. Been a while since I did one outside the simulator (or in it ... I'm due for my next trip to Simuflite soon).

FL270

#### Tired

##### New Member
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We let the speed bleed back at FL430 to M0.70 before starting the descent, then at idle power.

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If you close the throttles at FL430 do you still have enough bleed air to keep the cabin alt from rising? I understand it doesn't matter at the speed descend in that example, but what about in a normal descent. One problem we have in the Falcon 20 is that if try to fly the max economy descent profile you will trigger the cabin alt warning if you idle the engines at the very top of the descent.

#### FalconCapt

##### New Member
[ QUOTE ]
If you close the throttles at FL430 do you still have enough bleed air to keep the cabin alt from rising? I understand it doesn't matter at the speed descend in that example, but what about in a normal descent. One problem we have in the Falcon 20 is that if try to fly the max economy descent profile you will trigger the cabin alt warning if you idle the engines at the very top of the descent.

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Yes, in the Falcon 50EX and 900EX we have a BASC (Bleed Air System Computer). It will regulate the bleed air system and modulate in HP (high-pressure) bleed air as necessary to maintain cabin pressurization and anti-ice functions.

You are right, the Falcon 20 is very lacking in excess bleed air, any significant reduction in power up high and that cabin would climb. The TFE731-40's and -60's have a LOT of excess bleed air, as a matter of fact, under normal operations with anti-icing off, the BASC never even taps into the HP bleed air, LP is sufficient.