The Alsim cabin pressure system has a programming flaw in it. The graphics are designed for a maximum 5.2 psi cabin pressure differential, but the emergency cabin outflow valve opens at 6.0 psi. As a result it is possible to experience a cabin altitude warning even though all system components are operating normally. In reality it should only possible to experience a cabin over pressure if the emergency outflow valve sticks closed.
The simulation on this page is programmed to work the way a cabin pressure system should work. In order to clarify the situation there is a button that allows you to switch between a 5.2 psi and a 6.0 psi system.
The simulation shows a schematic representation of how a cabin pressure controller works. To understand the simulation you need to grasp the following:
The yellow lines represent a vacuum source, provided by a and engine driven pump or venturii
The red arrows (there really should be two) represent high pressure air bled from the engine compressor section
You may safely assume that the Air Inflow is adequate to pressurize the cabin even with the engines at idle.
The emergency outflow valve (lower valve on right side) is held closed by a spring calibrated to the maximum cabin pressure. This valve will be PUSHED OPEN by cabin pressure if the max differential is exceeded. Nothing can stop this short of a stuck valve. Therefore the cabin cannot possibly over pressurize.
A dump valve can be opened to "suck" the emergency outflow valve open. This valve is opened automatically when weight is on the gear. The pilot can also open it with the dump switch. The dump switch does not work in this simulation. Note that a pilot would need to dump the cabin for emergencies such as smoke in the cabin.
The main outflow valve is held closed by a spring. This valve can only be opened by "sucking" it open.
Given point 6, it is necessary for the cabin pressure controller to have a source of vacuum. This vacuum can be used to open the outflow valve, which will allow air to leave the cabin and consequently reduce cabin pressure (results in a cabin climb.)
A valve on the vacuum line opens once weight is off the gear providing vacuum to the controller.
The cabin controller has a diaphragm in it that will expand if cabin pressure increases and shrink it it decreases.
As the diaphragm expands and contracts it retracts or inserts a needle valve into the vacuum source.
The result of 9 and 10 is that if the diaphragm expands, because cabin pressure it too high, the needle valve opens resulting in a vacuum forming that opens the outflow valve, which results in the cabin pressure dropping.
On the other hand: if cabin pressure is too low the needle valve will be firmly inserted in the vacuum source. Thus the pressure in the controller will remain high and the outflow valve will remain closed. This results in the cabin pressure increasing.
The simulation assumes a ground altitude of 2000 agl. This value cannot be changed
Instructions:
On the cabin pressure controller set cruise altitude to 1000' above intended cruise altitude. For example if you will fly at FL220 set 23,000 feet.
If you set 23,000 feet cruise altitude you will have a cabin altitude (on outside of scale) of 6800 with the 5.3 psi system or 5500 with the 6.0 psi system
Set cabin rate controller to desired rate of cabin climb. This must be estimated based on the time it will take to climb to FL220. For example if it will take 10 minutes for the climb, given that the cabin must climb from 2000 to 6800 (5.2 psi case) the cabin rate must be at least 480 fpm. It would be smart to set a margin here, say 550. NOTE that the higher the max differential the lower the cabin rate can be. Make sure you understand why.
Now click on the "+" key beside the VSI to establish the desired rate of climb.
Note that immediately upon leaving the ground the electrically powered valves switch closing the dump valve and opening the valve that sends vacuum to the pressure controller.
Notice that cabin pressure begins to decrease, but pressure differential increases. In the simulation this shows on both the cabin altitude gauge (as in a real airplane) and in the lower right corner. Remember that a drop in cabin pressure shows on the cabin altitude gauge as an increase in cabin altitude.
In the lower right corner you can see cabin pressure dropping, but atmospheric pressure drops faster. I.E. the cabin altitude is climbing slower than the airplane is climbing. The difference is the cabin differential.
Notice that the pressure controller is experiencing a vacuum ( it is yellow) which means the outflow valve is held open, which allows the cabin to climb.
If you change the cabin rate controller you adjust a second needle valve. Closing the cabin rate needle valve determines how quickly the diaphragm can expand or contract. This in effect fine tunes how much vacuum is applied to the outflow valve.
As the airplane reaches 22,000 feet stop the climb by clicking the "0" key beside the VSI.
Catching the Cabin
If you set the cabin rate too low you will reach maximum differential before you reach your cruise altitude. If this happens the emergency outflow valve will open and the cabin climb rate will jump to approximately match the aircraft rate of climb. To see this:
With the airplane established in cruise, as described above, click on the plus key beside the VSI to establish a climb.
As the airplane climbs above the set cruise altitude the cabin reaches max differential and the emergency outflow valve opens.
At this time observe the cabin rate of climb.
Setting Cabin Pressure for Descent
Prior to descending cabin pressure (outer scale) should be set to 500 to 1000 feet above ground elevation. In this simulation set cabin altitude to approx 3000
The setting from step 1 ensure that the cabin will reach zero pressure differential when the airplane is on short final for landing. From this altitude to ground level the airplane will descend unpressurized.
With the system set as described set a suitable cabin rate of descent. If you set this too low the cabin will not descend quickly enough and will thus reach zero pressure differential too soon. The result will be a prolonged unpressurized descent with the cabin descending at the same rate as the aircraft. That would likely be uncomfortable for the passengers, so try to avoid this.
Note that in real life operations you normally level off several times during a descent. This increases the total time for the descent so you can set a comparatively low rate of cabin descent. Generally 300 fpm is optimum for passenger comfort, but with a 5.2 psi system you will likely need a higher rate in many cases.
Stuck Emergency Outflow Valve
If you click on the emergency outflow valve it will stick. It turns red to indicate that it is stuck. You can "stick it" in the open or closed position.
Try sticking the valve closed then climb above the selected cruise altitude. The cabin will over pressurize and an emergency annunciation "CABIN PRESSURE" will appear. If this happened in a real airplane it would be a major emergency and would require immediate descent. If the cabin continued to pressurize an attempt to dump the cabin should be made. If you do not to the above the simulation will reward you with the message that the cabin has exploded.
The other situation is to "stick the valve" open. Do this by putting the airplane on the ground before failing the valve. Now the valve will not close when the airplane takes off and the result will be an unpressurized cabin. After setting up this situation try climbing above 10,000 feet. The cabin altitude will equal the aircraft altitude. When cabin altitude exceeds 10,000 a cabin altitude warning will be annunciated. Note that the Alsim annunciation is incorrect for this situation.
