Page 22 - August 2015 Volume 9, Number 8

and we again have the runaway cabin dive after liftoff ... just as we did with no vacuum at all. The only difference between these two situations is that now the Dump switch will work. So we have two methods for depressurization: Dump – allowing vacuum to suck the Safety Valve wide open – or turn off the Bleed Air Valve switches and allow the cabin leak rate to gradually depressurize the airplane.
The next malfunction I will discuss is the Dump Solenoid valve itself. Like the Preset Solenoid valve, this receives power when the airplane is on the ground but is unpowered after liftoff. However, it is the opposite type of valve as compared to its Preset cousin: Normally-Closed (N.C.), and using electric power to go open.
If a wire to this valve comes loose in flight, we will observe nothing amiss. It remains in its de-energized, closed state. Only if we chose to move the Control switch to Dump – perhaps responding to heavy smoke in the cabin – would we find anything unusual when no dumping took place.
Ah, but after we land...now there is an interesting and potentially dangerous situation!
On the ground, both the Preset and the Dump solenoid valves should be energized, each going to their proper positions: The Preset Solenoid going closed, preventing any vacuum from reaching the Controller, and the Dump Solenoid going open, allowing vacuum to reach the Safety Valve and suck it wide open.
If the Dump Solenoid has a loose wire and does not energize like it should, then neither the Outflow nor the Safety Valve can receive vacuum and they both go to their spring-loaded, closed position. If the Bleed Air Valve switches are on, air is now flowing into an essentially closed box and we will begin gaining air mass in the pressure vessel, meaning that cabin pressure is going up and cabin altitude is, consequently, going down. Although this is an unregulated cabin dive, it is not as severe nor as noticeable as the runaway ∆P after takeoff, since the engines are at Idle and not much air is being supplied by the Flow Packs. More than one crew has taxied in after landing with the cabin slowly re-pressurizing yet did not notice it due to the rather gradual rate of cabin descent.
The shutdown is completed, someone steps back to open the cabin door, wonders why the release button is a little more stiff than normal, pushes it harder, rotates the handle, and finds himself sailing through the air onto the hard tarmac, having been unceremoniously ejected from the pressurized cabin. Realize that even 0.5 psid will cause the door to experience about 700 pounds of opening force!
Although preventing the cabin from re-pressurizing as we taxi in is not the primary reason for turning off the Bleed Air switches prior to shutdown, it is one
20 • KING AIR MAGAZINE
more advantage of doing so. Some pilots make a habit of always opening their cockpit vent window prior to heading back to open the cabin door, to know for sure that no ∆P remains. That may be a little excessive, but it surely isn’t a bad idea! Another method is simply to check the ∆P gauge and make sure it’s showing 0 psid before anyone operates the door.
The next pressurization abnormality I will mention is one that I’ll wager a lot of you have experienced. It’s not dangerous, just irritating, and somewhat uncomfortable. This phenomenon is almost exclusively experienced in the E90- and F90-series, later members of the C90- series, as well as in the 100-series.
There you are, enjoying a satisfyingly high ground speed as you descend near the barber pole redline airspeed on this smooth flying day. Then, boom! What was that?! Why is the cabin showing a dive of 2,000 fpm or more! Even after things stabilize, the cabin is still descending rapidly and the rate knob on the Controller has no effect. Dang!
You just joined the “I Blew Open the Ram Air Door” club. The ram air door that allows outside air to get into the pressure vessel – via the air conditioner’s evaporator plenum beneath the avionics bay on the left side of the nose wheel well – is normally prevented from opening by three things. First, there is a spring. Second, there is an electro-magnet helping keep it closed. Third, in most cases, there is enough ∆P to also force it closed. Yet when the airspeed is near redline – and the F90, with its higher VMO is the most notorious offender here – while ∆P is close to zero, the ram air force overcomes the spring and the magnet. That sudden inrush of outside air surely does dive the cabin!
Although you could avoid this by flying slower, who wants to do that on these rare smooth-air days?! Instead, you must make sure that ∆P doesn’t get too low – like below 1 psid – while zooming along near redline. How do you do this? By keeping an eye on the pressurization indicators during your descent and, specifically, making sure you are using sufficient cabin rate of descent so that the airplane’s altitude is not catching up to the cabin’s altitude. It’s having the airplane “catch the cabin” that is causing the problem.
It is tempting to use too low of a cabin rate-of-descent in an attempt to treat the passengers’ ears as gently as possible. That’s a great goal, but we must not overdo it. Unless you use a 400 to 500 fpm rate of cabin descent, there is an excellent chance that you may experience the irritating situation that we are discussing. There are exceptions to every rule – i.e., if you are landing at Aspen or Lake Tahoe, the cabin has so little altitude to lose that a low cabin rate-of-descent will probably work out fine – but sticking with the 400 to 500 fpm rate is almost always safe and not problematic for most passengers’ ears.
AUGUST 2015