There I was, shortly after takeoff, gear up, climbing out. While performing the after-takeoff checklist, I noticed that the cabin rate of climb was the same as the aircraft rate of climb. The cabin altitude was climbing with the aircraft altitude, hmm. No, I didn’t forget to turn the bleed air valves on, at least not this time. So, what gives?
To pressurize the cabin, we need air coming into the cabin – inflow – as well as some way to allow air to leave the cabin – outflow.
A common characteristic in all King Airs is the fact that they do not have a tight cabin. They tend to have high leak rates. Is this a concern? Not necessarily. Can the cabin maintain maximum differential when using only one flow control unit? With both bleed air valves open, can I reduce the power back until I hear the gear warning horn, without the cabin starting to climb? If the answer is yes to both questions, then I am happy with my King Air. However, if the answer is no to the questions, then I may have one or two weak flow control units (inflow) combined with an excessive leak rate (outflow), which is causing the pressurization problem.
Too much outflow, not enough inflow. Most pressurization problems in the King Air result from inflow and/or outflow issues, not the pressurization controller itself.
The following discussion does not include King Air models that have supercharges (straight 90, A90 or B90). What we will be discussing are the rare pressurization problems that could exist even if the cabin is tight and we have strong flow control units.
The ejector vacuum line becomes detached from the throat of the ejector
The first example of a rare malfunction involves the ejector vacuum line detaching from the throat of the ejector.
The normal outflow valve and the safety outflow valve, in the back of the cabin, need vacuum to open. If vacuum is lost due to the vacuum line disconnecting from the ejector, the internal spring wins and the outflow valves “fail” to the closed position. No more outflow! With the bleed air valves open, we are jamming air into a sealed container, otherwise known as the pressure vessel. If this occurred on takeoff, your ears surely would feel it!
High power setting, lots of inflow and no outflow. I know, I’ll just dump the cabin and that should stop the cabin from diving to a lower altitude. Nope, no change. The safety outflow valve needs vacuum to open. With no vacuum, there is no dump mode. The cabin differential will increase like a balloon ready to pop. Looks like we need to stop the inflow. By turning off the bleed air valves, we can let our leaky King Air do what it does best: leak air out of the pressure vessel. This could take more than 20 minutes to completely depressurize.
The preset solenoid fails closed
The next malfunction has to do with the preset solenoid. Prior to takeoff in most King Airs, we normally set our pressurization controller to 1,000 feet above our cruise altitude. Today’s setting equated to a desired cabin of 8,000 feet.
Normally, the preset solenoid blocks the vacuum to the normal outflow valve on the ground preventing the controller from trying to drive the cabin to higher altitude. This allows us to set the pressure controller on the ground. Once the airplane leaves the ground, the preset solenoid opens, allowing vacuum to modulate the normal outflow valve. We see the cabin climbing at a slower rate than the aircraft, and the rate of climb can be controlled using the rate knob on the pressure controller.
With a failed preset solenoid in the open position, the pressure controller will try to drive the cabin to that higher altitude as the controller is set during the before-takeoff checklist. The normal outflow valve would open (using vacuum) and would not start modulating until the aircraft reached the desired cabin altitude set in the controller. The cabin altitude is never permitted to be higher than the airplane altitude. If the cabin did get higher than the aircraft, there would be a negative differential pressure inside the cabin. The outflow valves are designed to open if there is negative differential pressure inside the cabin to prevent damage to the pressure vessel. During this specific malfunction, the outflow valves would remain open until the aircraft altitude matched the desired cabin altitude. The result is the airplane is essentially unpressurized during climb.
To recap: If the preset solenoid does not energize due to a broken wire or bad solenoid, it will remain open. Now vacuum drives open the normal outflow valve while on the ground. After takeoff the airplane will climb essentially unpressurized until it climbs through the desired cabin altitude, which was set prior to takeoff in the pressure controller. After this, the cabin will pressurize as normal. The aircraft altitude is now above the cabin altitude and the normal outflow valve will be allowed to work correctly. It is easy to think that the problem went away; however, during the next takeoff, you will experience the same issue. Don’t buy a new pressurization controller before you have the mechanic check the preset solenoid.
The preset solenoid gets stuck closed
If the preset solenoid can get stuck open, then it can also get stuck in the closed position. If this happens on takeoff, no vacuum will get to the normal outflow valve. It will remain closed during takeoff. The safety outflow valve closes after liftoff, as it should. We see the cabin in a dive. Both outflow valves are closed at the same time. With lots of inflow and a high-power setting for takeoff, the differential is quickly getting high. The dump mode will operate when the dump switch is moved to the dump position. Vacuum will be allowed to open the safety outflow valve. Ouch my ears! I may choose to close one or both bleed air valve switches and wait for the cabin to leak down slowly. That’s much easier on the ears than dumping the cabin.
The dump solenoid fails closed
The next malfunction involves the dump solenoid. It receives power when the airplane is on the ground to allow vacuum to open the safety outflow valve. After liftoff, the dump solenoid is deenergized, which causes it to close. This prevents vacuum from getting to the safety valve. Due to the loss of vacuum, the safety outflow valve closes. Now the normal outflow valve, in conjunction with the pressure controller, can control pressurization of the cabin.
During flight, if a wire comes loose from the dump solenoid, we will not notice any issues. However, once we touch down, the dump solenoid will not open and the safety outflow valve will remain closed on the ground. The normal outflow valve did its job and closed after touch down. Both outflow valves are now closed on the ground. If we left our bleed air valve switches open after landing, the cabin would start to pressurize. This will happen slowly due to lower power settings during taxi. After we park and try to open the cabin door it may be hard to push the red button prior to turning the handle to open the door. If you are successful in pushing the red button and turning the handle, that door will blow open rapidly, possibly pulling you with it and sending you out head first onto the ramp.
Another bad outcome could be the door blowing open so fast that it smashes into someone on the outside. Either way, bad things happen when the cabin is pressurized on the ground. Even very low differential pressure showing on the gauge makes a big impact. About 0.5 psid on the cabin door equates to about 700 pounds of force trying to open that door.
The first preventative action to this potential poor ending to a flight is to turn off both bleed air valves after every landing. Second, verify the differential pressure gauge is zero prior to letting anyone open the cabin door. Third, if you feel more resistance than normal when pushing the red button on the cabin door, don’t force it. It is trying to let you know that the cabin still has some differential pressure. You will have to wait for the cabin to leak down.
If you want to be really sure the cabin is depressurized, open the storm window in the cockpit. Some people open the storm window after every landing to ensure the cabin is depressurized, but it is not required.
The ram air door blows open in flight
I hear about this malfunction mostly from pilots flying E90 and F90 models, along with later models of the C90 and 100 lines. The story goes like this: “I was enjoying the barber pole high-speed descent when all of a sudden – pow! I hear a reverberation and the cabin is showing a big dive of more than 2,000 fpm!”
The ram air door has been inadvertently blown open. Many pilots who fly these models have achieved this unintentional milestone. The ram air door is what allows outside air to enter the cabin. Looking at the left side of the nose, you will see a vent opening where ram air can enter. The ram air door is normally held closed by three things: a spring, an electromagnet and differential pressure. As the airplane descends, the differential decreases. As the differential approaches zero, the potential of blowing the ram air door open increases.
During a high-speed descent, especially in an F90 with higher Vmo, there could be enough ram air force to overcome a weak spring and electromagnet, especially in conjunction with low differential pressure. Once the ram air door is blown open, there is a sudden rush of ram air that causes the cabin to dive. Good luck getting it closed. You will most likely have to wait until you get on the ground before it will close.
How can you avoid this? Don’t let the differential get below 1 psid. Slowing down a bit, prior to getting too low, will not only help to prevent blowing the ram air door open but it will also help when conducting a stabilized approach.
The petcock drain valve is left open
The final unusual pressurization malfunction has to do with a petcock drain on the right side of the baggage compartment just above the floor. It is hidden behind an access panel in the upholstery.
This valve is in a low point of the vacuum line between the pressure controller and the normal outflow valve. If this valve were left in the open position accidentally, then the controlled vacuum coming from the pressure controller would not regulate the normal outflow valve properly. Usually, this malfunction shows up as a runaway differential pressure. This can be remedied by closing the bleed air valves when you are ready to depressurize for landing. Sometimes it shows up as a “stuck” altitude, meaning the pilot could not get the cabin altitude to change using the pressurization controller.
In closing, it is possible for a pressurization controller to fail; however, there are many other issues that affect the cabin pressurization. Besides the rarer malfunctions listed in this article, most pressurization problems are the result of not enough in flow or too much out flow (leaks).