Are you familiar with the title’s term, “Catching the Cabin?” It usually refers to the action, in a descent, of having the airplane’s altitude match the cabin’s altitude. Why does this happen? Three reasons come to mind.
First, the pilot forgets to set the cabin altitude properly for the landing condition. Especially when flying as the sole pilot, with no one cross-checking your actions, it is relatively easy to miss the step on the descent checklist of setting the pressurization for landing. Instead, the pressurization controller (remember, it is merely a governor of cabin altitude) remains at the setting used for cruise, say at maybe 7,000 feet. If the controller is overlooked, then the cabin remains at 7,000 feet. As the airplane descends, the difference between inside and outside pressure decreases, of course. That is, Differential Pressure (ΔP) becomes less. Since it is impossible to have a negative value of ΔP, that means the cabin can never be higher than the airplane. So, when the airplane reaches the 7,000 feet in this example, ΔP becomes zero, the outflow and safety valves are pushed open, outside air flows into the cabin, and the cabin descends right along with the airplane. The occupants suddenly experience the airplane’s rate of descent, perhaps 1,500 fpm or even more. Hello, sinus pain!
The second reason why the airplane altitude matches the cabin altitude is because the rate of cabin descent was set too low on the pressurization controller’s rate knob. Let’s work a math problem together: Suppose in your C90B, the cabin was at 7,000 feet as you cruised at 22,000 feet. You are landing at Sea Level (SL). An unrestricted descent from FL220 to SL at 1,300 fpm will take about 17 minutes. For the cabin to get from 7,000 feet to SL in 17 minutes requires a rate of descent of about 412 fpm. If the rate knob is set for 300 fpm – perhaps in a misguided attempt to be kinder to the passengers’ ears – then at 1,500 feet the airplane will “catch the cabin” and the remaining 1,500 feet of descent will again see the passengers and their ears experiencing the rate of descent of the airplane.
The third and last reason for “catching the cabin” in a descent is simply coming down at a much greater rate than usual. Doing an emergency descent at, say, 5,000 fpm – maybe due to a passenger having a medical emergency – will take about 4.4 minutes from 22,000 to SL. For the cabin to get from 7,000 feet down to SL in that same amount of time equates to a little over 1,500 fpm. Since it’s unlikely that the pilot(s) will dial in such a high rate, it is probable that again the airplane will “catch the cabin” … leading to the last portion of the descent being unpressurized with the 5,000 fpm descent rate. Ouch!
The 90- and 100-series of King Airs face another inconvenience in the above situations. Remember that it is quite common for the ram air door on the lower left side of the nose to be blown open when high airspeed and low ΔP exist simultaneously. Three factors normally keep the door shut: (1) spring tension; (2) an electromagnet; (3) Differential Pressure (ΔP). However, it appears that the spring and the magnet are often insufficient to overcome the dynamic air pressure caused by high airspeed when there is no ΔP.
The forcing open of that door leads to the disconcerting outcome of rapid cabin pressure fluctuations … rapid cabin dives followed by rapid cabin climbs. The sudden addition of outside ram air dives the cabin down but the resultant higher cabin pressure then closes the ram air door, the cabin starts to climb as it loses the extra ram air input, the door then blows open again and the whole process keeps repeating until the airspeed is slowed or P again becomes positive.
The 200- and 300-series have a different ram air door system. You will notice that the NACA scoop on the lower left side of the nose no longer exists. It has been replaced by a door on the forward end of the air conditioning’s evaporator chamber, which is now on the right side of the nose. The NACA scoop was no longer needed since the inlet to the air conditioning’s condenser is on the right side also. By adding a hole in the floor that the condenser sits on, ram air becomes present at the door. The more convoluted passage of air must tend to decrease its pressure since the blowing open of the door at high airspeeds is a nonexistent phenomenon. As a side note, the 200-series and 350-series with the Keith environmental system have no ram air inlet at all!
With the exception of the emergency descent, there are two simple actions that will assure you will not “catch the cabin” during the descent. First, set a sufficient rate of descent with the rate knob. Rarely will 400-500 fpm not be enough to guarantee the cabin will get close to field elevation before you get there. Except when dealing with a severe head cold and stuffy sinuses, rarely will anyone be uncomfortable at these rates of descent.
Second, as I have discussed in The King Air Book and previous articles, why wait until nosing over in the descent to set the pressurization for landing? In these days of nonsmokers and clean outflow valves, there is no problem whatsoever in running at maximum ΔP in cruise. Thus, move the “Pressurization – SET FOR LANDING” step from the Descent checklist to the last step of the Cruise checklist. That way, it cannot be overlooked in the busyness of the descent.
Most all of what I have written above is “old hat” for pilots of pressurized airplanes. But now let me present another case of “catching the cabin” that is not nearly as common nor well-known.
In this scenario, it’s the cabin, climbing, catching the airplane instead of the airplane, descending, catching the cabin. I recently received an email concerning this phenomenon and then had a telephone conversation with the pilot who was involved. After we had discussed the situation a bit, I asked him if he was based in the Dallas, Texas, area. “Yes! At Ft. Worth!” was his answer. In my experience, the Dallas-Ft. Worth metroplex almost always keeps turboprops down at relatively low altitudes for a l-o-n-g time when departing.
Let’s again assume that our controller’s Cabin Altitude knob has been set for a 7,000 foot cabin before we departed. Soon after takeoff we observe the cabin is climbing at 500 fpm as we have set. Perfect!
But then ATC tells us to level at 3,000 feet. Friends don’t let the cabin catch you!
If ATC keeps us down at 3,000 feet for four minutes or longer, the cabin will catch up to us as it continues its attempt to reach 7,000 feet. In the olden days of PT6A-20-powered A90s, B90s and early C90s, their speeds were low enough that rarely were ram air doors blown open in this situation. It took high-speed descents to do it. But now, with -21s and -135As so prevalent and with a Vmo of 223 knots – even higher on the F90 – versus the old 208-knot redline, the newer airplanes definitely have the ability to blow the ram air door open in level flight.
So, there you are, impatiently waiting at 3,000 feet for the order to continue climb, when suddenly you – and your passengers – feel some major pressure fluctuations! You see the cabin’s VSI fluctuating wildly, from 2,000 fpm down to 2,000 fpm up. Welcome to the “I blew open the ram air door” club!
Now at long last ATC assigns you to, say, 10,000 feet. Being so frustrated with the wasted time at 3,000 and having built up a great deal of airspeed, you pitch that sucker up to 10 or 15 degrees and convert some of that airspeed into altitude. Now what?! Why is the pressurization not working?! Darn it, ΔP is staying at zero and the cabin is climbing as fast as we are! What the heck is wrong now?!
Nothing. You see, although the “real” cabin could not go above you and hence remained at 3,000 feet, the controller was still trying to raise the cabin to 7,000 feet at 500 fpm. What I call a “phantom” cabin exists above you. If held at 3,000 feet for five minutes, this phantom cabin – where the controller is trying to put the real cabin – is now passing 5,500 feet (3,000 + 500 fpm x 5 minutes). Only when the airplane and the real cabin catch up to the phantom cabin will the controller finally be able to operate correctly. About the time you are making the decision to try and sell this lousy King Air because of its pressurization problems the dang thing starts to work perfectly … and does so for the rest of the flight!
These low-altitude, long-duration, level-offs are so rare that it is easy to forget or overlook what needs to be done to prevent the problems I have just described. Here’s the key: Don’t let the cabin catch up to you in its climb.
As soon as you sense that the low altitude level-off is going to last longer than desired, crank the rate knob to the counterclockwise stop, as low as it will go. If the cabin stops ascending at 500 fpm and only creeps up at about 100 fpm, it prolongs the time before ΔP nears zero and the problems manifest themselves. Once you are cleared higher, then reset the rate knob for your normal 500 fpm or so.
But in an extreme case, maybe even a very slow-climbing cabin eventually catches up to the airplane. We cannot let that happen without asking for discomfort. If you see the cabin getting within about 1,000 feet of the airplane – and that means ΔP will be nearing about 0.5 psid – then take the cabin altitude knob on the controller and dial it down to 1,000 feet or so below you (2,000 feet in our example). That will maintain the 0.5 psid ΔP and prevent the ram air door from opening. It also means there will be no phantom cabin above you. As ATC finally permits the climb to continue, then reset the rate and altitude knobs to the normal settings.
To summarize, there are only two times during normal operations that you want to see ΔP at zero: First, at and right after takeoff; second, just before landing. At all other times, don’t allow the airplane to “catch the cabin” while descending, and don’t let the cabin “catch the airplane” while climbing. Got it?!