737 Pressurisation Panel
Thread Starter
Join Date: Nov 2005
Location: Planet Earth
Posts: 7
Likes: 0
Received 0 Likes
on
0 Posts
737 Pressurisation Panel
On the Classic 737 pressurisation panel, there are three altitudes that must be input prior to departure. One is the flight alt, which is obviously the altitude the route will be flown at. The second and third ones are the landing alt and the cabin alt. The cabin alt is the alt you are currently at, at the departure airfield, and the landing alt is the alt of the destination airport. However one of these figures are set -200 ft, but I'd like to know which one, and why?
Thanks.
If I've made any other mistakes please point them out.
Thanks.
If I've made any other mistakes please point them out.
Per Ardua ad Astraeus
Join Date: Mar 2000
Location: UK
Posts: 18,579
Likes: 0
Received 0 Likes
on
0 Posts
It is a Boeing 'requirement' that the cabin is positively pressurised during take-off and landing - I cannot remember which 'reason' - either ?to give it a structural stiffness? or to lessen the effects of pressure changes on pax. The -200ft gives, I think from memory, something like +0.13 psi. NG Boeings do this automatically.
Join Date: Aug 2005
Location: SEA (or better PAE)
Posts: 215
Likes: 0
Received 0 Likes
on
0 Posts
In case of increased stiffness once the high-blow test is done, prior to a/c delivery there is no benefit in additional pressurization.
So stiffness is not the reason for your question.
Cheers,
So stiffness is not the reason for your question.
Cheers,
Join Date: May 2006
Location: Infinity and Beyond
Posts: 82
Likes: 0
Received 0 Likes
on
0 Posts
The landing alt is set to aerodrome level amsl minus 200ft to help eliminate the pressure bump at take off and to also ensure that the aircraft is fully depressurised upon landing.
The cabin alt parameter is only used in stby mode where you set the desired cabin alt ie 8000'
have a look at www.b737.org.uk which gives a breakdown on the B737 classic and NG systems.
Hope this helps
The cabin alt parameter is only used in stby mode where you set the desired cabin alt ie 8000'
have a look at www.b737.org.uk which gives a breakdown on the B737 classic and NG systems.
Hope this helps
Per Ardua ad Astraeus
Join Date: Mar 2000
Location: UK
Posts: 18,579
Likes: 0
Received 0 Likes
on
0 Posts
and to also ensure that the aircraft is fully depressurised upon landing.
"actually this ensures that the cabin IS pressurised on landing! It is the opening of the outflow valve that depressurises the cabin."
Correct. That is why the cabin-crew can't open the doors if you forget the FLT-GND switch taxying in. We have all done it, especially on a mixed fleet of CPCS & DCPCS! Cracking your DV window open will assist the process.
S&L
Correct. That is why the cabin-crew can't open the doors if you forget the FLT-GND switch taxying in. We have all done it, especially on a mixed fleet of CPCS & DCPCS! Cracking your DV window open will assist the process.
S&L
Join Date: Jul 2006
Location: Brisbane, Oz
Age: 82
Posts: 46
Likes: 0
Received 0 Likes
on
0 Posts
Further to AEUENG's comment, the pressure 'bump'/surge occurs as you push the lower rear mounted outflow valve towards the runway surface on rotate, which sits in the underwing higher pressure airflow, amongst other effects.
The standby mode now-a-days is just what we used to use day in and day out, back when pressurization was a novelty. a.k.a Viscount fun times. You had to think then, and the ability to think 'how high am I? _what has to happen to the valve to make the cabin pressure increase and so make us/the cabin go 'down' ' was a definite benefit.
The standby mode now-a-days is just what we used to use day in and day out, back when pressurization was a novelty. a.k.a Viscount fun times. You had to think then, and the ability to think 'how high am I? _what has to happen to the valve to make the cabin pressure increase and so make us/the cabin go 'down' ' was a definite benefit.
Join Date: May 2005
Location: Standing at P37
Posts: 187
Likes: 0
Received 0 Likes
on
0 Posts
TAKEOFF MODE
With the airplane on the ground and the FMC indicates that ground
speed is >65 knots, the pressure controller enters takeoff mode and
the outflow valves move towards closed position to pressurize the
cabin to 50 feet below the takeoff field elevation at a nominal rate of
500 feet/minute, to minimize the effect of pressure bump at airplane
rotation i.e. improve passenger comfort.
The above is quoted from the 747-400 Engineering Training notes. I know it's a different type than you asked about but the reason for the initial pressurization is the same- i.e passenger comfort! JenCluse beat me to it - his statement is the exact reason. The outflow valves are at the rear of the fuse and the rotation of the aircraft at lift off moves the outflow valves 'down' and/or they descend in real terms - whilst at the same time the front of the aircraft (where the important people sit i.e first/business class/tech crew) climbs and/or ascends in real terms. This transient difference in altitude from the front to the back (combined with the actual increase in altitude of the aircraft as a whole) can't be catered for by the cabin pressure controllers. Thus to overcome this, the initial pressurization removes the pressure 'bump' that would otherwise be experienced with the accompanying discomfort to sensitive ears. The 737 doesn't quite have the advanced cabin press controllers and the interface to the FMC's of the later aircraft so this iniotial pressurization is accomplished as you say by selecting minus 200ft.
With the airplane on the ground and the FMC indicates that ground
speed is >65 knots, the pressure controller enters takeoff mode and
the outflow valves move towards closed position to pressurize the
cabin to 50 feet below the takeoff field elevation at a nominal rate of
500 feet/minute, to minimize the effect of pressure bump at airplane
rotation i.e. improve passenger comfort.
The above is quoted from the 747-400 Engineering Training notes. I know it's a different type than you asked about but the reason for the initial pressurization is the same- i.e passenger comfort! JenCluse beat me to it - his statement is the exact reason. The outflow valves are at the rear of the fuse and the rotation of the aircraft at lift off moves the outflow valves 'down' and/or they descend in real terms - whilst at the same time the front of the aircraft (where the important people sit i.e first/business class/tech crew) climbs and/or ascends in real terms. This transient difference in altitude from the front to the back (combined with the actual increase in altitude of the aircraft as a whole) can't be catered for by the cabin pressure controllers. Thus to overcome this, the initial pressurization removes the pressure 'bump' that would otherwise be experienced with the accompanying discomfort to sensitive ears. The 737 doesn't quite have the advanced cabin press controllers and the interface to the FMC's of the later aircraft so this iniotial pressurization is accomplished as you say by selecting minus 200ft.
Join Date: Apr 2004
Location: England
Posts: 67
Likes: 0
Received 0 Likes
on
0 Posts
737 Classic.
Before Take off, FLT selected gives minus 200' of departure field. This reduces noise, helps stops fumes ingressing into cabin and helps lessen the pressure bump after take off. CRZ alt selction is to ensure a max diff is acheived when at final crz alt. On landing the press controller drives the pressurisation to minus 200' from the alt selected in the landing alt window. This is for the same reasons as above. The sby window is only used when sby mode is selcted. You just have to change the selections in the window in line with the above instead of the auto controller sensing the aircraft flt path and doing it for you.
One point though. Minus 200' equtes to 1.25 psi (i think...), which is I think the max pressure allowed under JAR ops (again, "I think") when an aircraft is on the ground. This is because all aircraft doors must be capable of being OPENED wilst on the ground and pressurised. This is to protect against flt deck incapacitation etc..... And I have forgot to go to gnd after landing
The first I knew about it was a breif rush of air rearwards when our number one muscled open the door.
I stand to be shot down in flames and corrected though........
Mr L.
Before Take off, FLT selected gives minus 200' of departure field. This reduces noise, helps stops fumes ingressing into cabin and helps lessen the pressure bump after take off. CRZ alt selction is to ensure a max diff is acheived when at final crz alt. On landing the press controller drives the pressurisation to minus 200' from the alt selected in the landing alt window. This is for the same reasons as above. The sby window is only used when sby mode is selcted. You just have to change the selections in the window in line with the above instead of the auto controller sensing the aircraft flt path and doing it for you.
One point though. Minus 200' equtes to 1.25 psi (i think...), which is I think the max pressure allowed under JAR ops (again, "I think") when an aircraft is on the ground. This is because all aircraft doors must be capable of being OPENED wilst on the ground and pressurised. This is to protect against flt deck incapacitation etc..... And I have forgot to go to gnd after landing
The first I knew about it was a breif rush of air rearwards when our number one muscled open the door.I stand to be shot down in flames and corrected though........
Mr L.
Join Date: Jan 2006
Location: Sussex, UK
Posts: 55
Likes: 0
Received 0 Likes
on
0 Posts
One additional factor that affects this:
the outflow valve is designed to control the cabin pressure for a positive pressure differential. If you wind up with a negative delta P, the valve isn't as good at controlling the reverse flow case. So the additional +0.5 / 1.25 psi or whatever your system is designed to use ensures that you avoid this situation.
the outflow valve is designed to control the cabin pressure for a positive pressure differential. If you wind up with a negative delta P, the valve isn't as good at controlling the reverse flow case. So the additional +0.5 / 1.25 psi or whatever your system is designed to use ensures that you avoid this situation.
Join Date: Jan 2004
Location: LPPT
Age: 58
Posts: 431
Likes: 0
Received 0 Likes
on
0 Posts
I know the "OFF SCHED DESCENT" light illuminates if the aircraft has descended before reaching the selected flight altitude. Do we get this amber light in case of slow decompression i.e. is it tied to the cabin alt / flight level differential? What is the real use of this warning light?
GD&L
GD&L
Join Date: Apr 2005
Location: Yellowknife N.W.T
Posts: 27
Likes: 0
Received 0 Likes
on
0 Posts
737 pressurization and Off Sched Descent
All very correct except that the Max Diff for T/O and Landing is .125 psi which according to my manual is an AFM limitation from Boeing. If this number was used because of JAR requirements etc. I do not know.
As for the OFF SCHED DESCENT light. It is connected to the actual A/C Pressure Alt and is only functional when using the Auto Mode. As the A/C climbs to within 1000’ (so the good book says) of the altitude set in the flight alt window of the Auto controller a rely closes or trips whichever you like to say that signals the controller to begin maintaining a constant cabin pressure differential of 7.45 or 7.8 psi. (there may be others but these are the max normal diff’s the controllers in our A/C are designed to maintain) If this rely does not get activated prior to the A/C starting a descent then the aforementioned light will come on which is telling you that the controller has started to schedule a descent back to the T/O field alt as memorized by the controller prior to T/O (it’s actually the cabin alt at T/O it remembers) In short it thinks you are in an emergency return to departure airport and thus relieving the crew of having to deal with changing the pressurization controller during times of high work load.
You can imagine what would happen to the diff pressure if you departed from an airport at sea level and were going to land at one at say 3000’ ASL or visa a versa and never achieved you selected flight alt. Ie: You would not have .125 psi diff. Thus the Warning / Information light.
It should be noted also that if at anytime during the flight the flight alt selector knob is depressed as if to change the flight alt in the window this function is lost and could result in over pressurization etc. at landing.
Now that I have rambled on about things you may already know. The answer to your question is no you will not get this light during a slow decompression. You would if the rely had not closed / tripped prior to you starting the descent no mater what the current cabin pressure.
Cheers all and hope this helps.
As for the OFF SCHED DESCENT light. It is connected to the actual A/C Pressure Alt and is only functional when using the Auto Mode. As the A/C climbs to within 1000’ (so the good book says) of the altitude set in the flight alt window of the Auto controller a rely closes or trips whichever you like to say that signals the controller to begin maintaining a constant cabin pressure differential of 7.45 or 7.8 psi. (there may be others but these are the max normal diff’s the controllers in our A/C are designed to maintain) If this rely does not get activated prior to the A/C starting a descent then the aforementioned light will come on which is telling you that the controller has started to schedule a descent back to the T/O field alt as memorized by the controller prior to T/O (it’s actually the cabin alt at T/O it remembers) In short it thinks you are in an emergency return to departure airport and thus relieving the crew of having to deal with changing the pressurization controller during times of high work load.
You can imagine what would happen to the diff pressure if you departed from an airport at sea level and were going to land at one at say 3000’ ASL or visa a versa and never achieved you selected flight alt. Ie: You would not have .125 psi diff. Thus the Warning / Information light.
It should be noted also that if at anytime during the flight the flight alt selector knob is depressed as if to change the flight alt in the window this function is lost and could result in over pressurization etc. at landing.
Now that I have rambled on about things you may already know. The answer to your question is no you will not get this light during a slow decompression. You would if the rely had not closed / tripped prior to you starting the descent no mater what the current cabin pressure.
Cheers all and hope this helps.
