|Quoting TOGA (Thread starter):|
What happens if the pilot at cruise altitude forgets to set the landing altitude on the pressurisation panel and a decent is commenced ?. Does the differential pressure increase and does the cabin altitude remain as it was at cruise level ?
On older jets like the JetStar, we had to manually set the pressurization controller before descent to the field elevation. At FL370, the cabin altitude was about 6500 feet at 8.9 psid and if we were landing at a high elevation airport like Cheyenne WY
which is about 6200 feet elevation, we did not have to reset the controller. During descent the cabin differential pressure would drop from 8.9 psid to 0 as we descended and landed. We normally used 500 feet per minute for the cabin rate of climb and descent which made for a very smooth cabin descent or climb.
If we were to forget to reset the controller for landing and were landing at a lower elevation airport, as the airplane passed 6500 feet the outflow valves would open wide and the airplane would then become unpressurized and the cabin would follow the airplane down with the same rate of descent.
Our normal procedure was to set the controller 500 feet below the elevation of the airport we were landing at and when we landed the outflow valves would open and there would be a slight bump up to field elevation and this served to help clear our ears, especially if we made a quick decent to landing.
Flying to out of the way non airline airports with little or no traffic ATC would usually give us a fast descent. One time after passing through FL180 on descent, it was a severe clear day and we had the airport it sight so we canceled IFR. We called the tower at 15,000 feet abeam downwind, we then flew a long downwind and turned a wide base at 8000 feet and landed. We were able to do this because there was no other traffic in the airport control zone. To do this we had 3 engines at idle, one up to 80 percent to maintain enough bleed air for cabin pressurization and the speed brake and approach flaps out so we were dropping like a brick. The field elevation was around 3000 feet and the cabin controller had already brought the cabin to field elevation so there was no problem.