Post by LOU on Sep 25th, 2011 at 8:53pm
The 727 and for that matter 707 old style (as in the CS model) pressure control panels were very simple to operate, but took a bit of art on the F/E's side to make smooth.
The concept is simple. Air In - Air Out.
The engines supply high pressure bleed air through a pressure control valve on the engine. The F/E uses the pressure control panel to send signals to the outflow valves to control the amount of pressure in the plane's cabin. In the drawing, I show the air as water, but the concept is the same. The differential pressure is the difference of the air pressure inside the cabin to outside the cabin, displayed as P.S.I.
Here is a look at a real 727 pressure control panel. The knob on the lower right is where you select the altitude you want the cabin to go to. In this picture the cabin is selected to something less than 1,000 feet below MSL and the little window in the center top of the instrument shows the airplane altitude at max differential pressure. It indicates about 19,200 feet. Max differential is just over 9 psi. The knob just above the cabin altitude select knob is the barometric correction for local pressure. The baro window at about 2 o'clock shows 28.35 inches of Mercury (pretty low outside pressure - standard is 29.92). The other knob on the lower left is the rate of change selector. The arrow indicates the position of the selector - in this case almost max rate. If you align the arrow with the white triangle under the word RATE it should give you around 500 FPM.
So, how does it work?
Here is a look at the Captain Sim 727 pressure panel. It consists of 4 parts. The manual pressure control on the lower right, the automatic pressure control, lower left and two gauges above. The gauge on the above right is the rate of climb or descent indicator (VSI) and the one to the upper left is a combined cabin altitude and cabin differential gauge.
This is how the F/E would operate this system.
Since there is no ground venturi switch on this basic system the panel is operated just a little different to avoid large pressure bumps. After engine start, one or both packs can be started for air conditioning before the doors are closed. Just before the doors are closed the pack should be turned off. The pack valves are designed to open and close slowly to help with squeezing the ears. If the plane had a ground venturi fan, it would hold the outflow valves open and really aid in controlling the pressure bump. But is doesn't so tough it out!
Once all the doors are closed - cargo and cabin - the F/E can turn on one pack at a time and give the system time to react. You could select an altitude, on the automatic panel, to something above the current field elevation, but there is little muscle in the system to react since the cabin and the plane are at the same altitude. As the pack valve opens, there will be a small down surge in the cabin pressure and this can be seen on the VSI. After the pressure settles down you can open the second pack valve. Now as the plane taxis out for takeoff any addition of thrust from the engines will result in a down surge in the cabin - nice system, Hun! Most pilots were aware of the pressure system and would try to avoid doing this to the F/E, since it also effected them as well. As you took the runway for takeoff the F/E would check the differential pressure to make sure the red placard was being followed. You are limited to a max differential pressure of .125 psi on the ground. This is so the doors can be opened in an emergency. As the power was increased for takeoff the F/E would move the rate selector to minimum rate and maybe turn the cabin altitude selector up above the field elevation, but there was not much the poor F/E could do at this point. There would be a pressure bump as the plane was rotated for takeoff and airflow over the outflow valves changed. Not much the F/E could do here either. As the plane started to climb the F/E would select the desired cabin altitude for cruise plus a few thousand extra feet to avoid getting too close to the automatic pressure relief of 8.4 psi. Once established in a climb the F/E would adjust the rate knob to obtain around 500 fpm climb of the cabin. If the plane was light and climbing fast, the F/E would have to increase the climb rate of the cabin so as no to get caught by the climbing plane. If that were to happen, the cabin would climb with the plane and all the passengers and the other two pilots would curse the poor F/E to tears. This last sentence was an example of the negative pressure relief system.
At top of descent the F/E would slowly start the cabin down to the destination airport field elevation. Again, you needed to keep the cabin ahead of the plane in descent or the same thing would happen and more cursing would ensue. If you took off from a sea level airport and your destination was Denver, you might not have to do too much with the cabin altitude as the cabin was probably very near the field elevation. To change cabin altitude either up or down required slow and careful movement of the cabin select knob to avoid the pressure bump. Most F/E's got very good at this system and kept everybody happy. If you had a leaky plane, idle thrust might not have enough bleed air to keep the cabin from climbing during descent. F/E's would ask the pilots to push up one of the throttles just a bit to get a little more bleed air and keep the cabin going down.
The manual controller worked just like the auto controller, but you had to apply more attention to the device since any power change resulted in a different rate, and you needed to keep an eye on the differential and cabin altitude since there was no automatic level off at the correct altitude. The manual controller and automatic controller are connected, but the manual controller had more muscle and would override the auto controller.
I hope this little explanation helps to understand the old fashioned pressure system. It was fun to make it work nice, but I for one was not unhappy to see it go either. :o
Lou
P.S. the latest version of the CS727 & 707 cabin pressure systems do not work as they should for whatever reason. A fix is in the plan.
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The concept is simple. Air In - Air Out.
The engines supply high pressure bleed air through a pressure control valve on the engine. The F/E uses the pressure control panel to send signals to the outflow valves to control the amount of pressure in the plane's cabin. In the drawing, I show the air as water, but the concept is the same. The differential pressure is the difference of the air pressure inside the cabin to outside the cabin, displayed as P.S.I.
Here is a look at a real 727 pressure control panel. The knob on the lower right is where you select the altitude you want the cabin to go to. In this picture the cabin is selected to something less than 1,000 feet below MSL and the little window in the center top of the instrument shows the airplane altitude at max differential pressure. It indicates about 19,200 feet. Max differential is just over 9 psi. The knob just above the cabin altitude select knob is the barometric correction for local pressure. The baro window at about 2 o'clock shows 28.35 inches of Mercury (pretty low outside pressure - standard is 29.92). The other knob on the lower left is the rate of change selector. The arrow indicates the position of the selector - in this case almost max rate. If you align the arrow with the white triangle under the word RATE it should give you around 500 FPM.
So, how does it work?
Here is a look at the Captain Sim 727 pressure panel. It consists of 4 parts. The manual pressure control on the lower right, the automatic pressure control, lower left and two gauges above. The gauge on the above right is the rate of climb or descent indicator (VSI) and the one to the upper left is a combined cabin altitude and cabin differential gauge.
This is how the F/E would operate this system.
Since there is no ground venturi switch on this basic system the panel is operated just a little different to avoid large pressure bumps. After engine start, one or both packs can be started for air conditioning before the doors are closed. Just before the doors are closed the pack should be turned off. The pack valves are designed to open and close slowly to help with squeezing the ears. If the plane had a ground venturi fan, it would hold the outflow valves open and really aid in controlling the pressure bump. But is doesn't so tough it out!
Once all the doors are closed - cargo and cabin - the F/E can turn on one pack at a time and give the system time to react. You could select an altitude, on the automatic panel, to something above the current field elevation, but there is little muscle in the system to react since the cabin and the plane are at the same altitude. As the pack valve opens, there will be a small down surge in the cabin pressure and this can be seen on the VSI. After the pressure settles down you can open the second pack valve. Now as the plane taxis out for takeoff any addition of thrust from the engines will result in a down surge in the cabin - nice system, Hun! Most pilots were aware of the pressure system and would try to avoid doing this to the F/E, since it also effected them as well. As you took the runway for takeoff the F/E would check the differential pressure to make sure the red placard was being followed. You are limited to a max differential pressure of .125 psi on the ground. This is so the doors can be opened in an emergency. As the power was increased for takeoff the F/E would move the rate selector to minimum rate and maybe turn the cabin altitude selector up above the field elevation, but there was not much the poor F/E could do at this point. There would be a pressure bump as the plane was rotated for takeoff and airflow over the outflow valves changed. Not much the F/E could do here either. As the plane started to climb the F/E would select the desired cabin altitude for cruise plus a few thousand extra feet to avoid getting too close to the automatic pressure relief of 8.4 psi. Once established in a climb the F/E would adjust the rate knob to obtain around 500 fpm climb of the cabin. If the plane was light and climbing fast, the F/E would have to increase the climb rate of the cabin so as no to get caught by the climbing plane. If that were to happen, the cabin would climb with the plane and all the passengers and the other two pilots would curse the poor F/E to tears. This last sentence was an example of the negative pressure relief system.
At top of descent the F/E would slowly start the cabin down to the destination airport field elevation. Again, you needed to keep the cabin ahead of the plane in descent or the same thing would happen and more cursing would ensue. If you took off from a sea level airport and your destination was Denver, you might not have to do too much with the cabin altitude as the cabin was probably very near the field elevation. To change cabin altitude either up or down required slow and careful movement of the cabin select knob to avoid the pressure bump. Most F/E's got very good at this system and kept everybody happy. If you had a leaky plane, idle thrust might not have enough bleed air to keep the cabin from climbing during descent. F/E's would ask the pilots to push up one of the throttles just a bit to get a little more bleed air and keep the cabin going down.
The manual controller worked just like the auto controller, but you had to apply more attention to the device since any power change resulted in a different rate, and you needed to keep an eye on the differential and cabin altitude since there was no automatic level off at the correct altitude. The manual controller and automatic controller are connected, but the manual controller had more muscle and would override the auto controller.
I hope this little explanation helps to understand the old fashioned pressure system. It was fun to make it work nice, but I for one was not unhappy to see it go either. :o
Lou
P.S. the latest version of the CS727 & 707 cabin pressure systems do not work as they should for whatever reason. A fix is in the plan.
Uploaded with ImageShack.us