Page 18 - Volume 12 Number 6

No, it is because the inflow of air ceases. (Electric power is needed to keep the flow packs open.)
Somewhat surprisingly, since it is rather complex, the AiResearch control system is quite reliable. The problem with an airplane that cannot maintain the cabin altitude selected is very rarely due to a bad controller. Instead, it almost always is caused by too little inflow or too much outflow or a combination.
Troubleshooting Pressurization Problems
You have discovered that your pressurization is not working as it should. For example, you cannot reach 6.4 – 6.6 psid ∆P, or you see the cabin starting to climb even though the power levers have been only slightly reduced. How can you find what’s wrong? How can you help your mechanic reduce his troubleshooting time? Here are some ideas that pilots can do in flight. Mechanics have their own and, sometimes, more accurate procedures to use.
First, you can make sure the controller is functioning properly in this manner: In level flight, set the controller’s cabin altitude for 3,000 or 4,000 feet below you. For example, fly at 10,500 feet with the cabin set for 7,000 feet. Now zoom up to 11,500 and then dive down to 9,500 without changing engine power. Does the cabin stay level as it should? Next, back in level flight, dial the cabin up to, say, 9,000 feet. Does it start climbing? Twist the rate knob to the minimum setting. Does the cabin rate of climb decrease to almost nothing? Now spin the rate knob to maximum. Does the cabin climb like a homesick angel? Next, dial the cabin down to a lower altitude and check the rate control again as it descends. In almost all cases, you will find that the controller is working perfectly. As I wrote above, it is a surprisingly robust piece of gear. By doing this test with a small difference between airplane and cabin altitude, ∆P is very low and thus the effect of excessive leaks or weak inflow will also be low.
Second, on a deadhead leg – so that passengers’ ears will not be subjected to uncomfortable pressure fluctuations – force ∆P to the maximum attainable by dialing the cabin altitude down to sea level while you
are up high, typically above FL180. When the cabin stops descending, note the indicated ∆P. (Write it down or, better yet, take a picture.) You have forced ∆P to its maximum attainable value and if it is not within 0.1 psid of the ∆P gauge’s redline, then you have identified a problem.
Move the left bleed air valve switch to the center, Envir Off, position. (It doesn’t matter which side you do first, but we’ll start with the left.) Take a video of the cabin VVI while you do this or at least note and record the peak cabin climb that takes place. Maybe it hits a peak, say, of 1,600 fpm. What should next happen is that the cabin will stop its climb, go into a descent, and return to the exact altitude where it began. The King Air should be able to maintain maximum ∆P even with only one flow pack supplying air. Can yours do that? It is not at all uncommon to find the cabin will not descend back to where it started. Let’s assume that is what we see here ... the cabin does not recover back to its starting altitude but keeps climbing at an ever- decreasing rate. This means either the still-operating flow pack is weak – lack of inflow – or the leaks are excessive – too much outflow – or a combination of both. When we finish this little test, we will know what the problem is.
Turn the left bleed air valve switch back on and give plenty of time for the situation to return to normal operation, with the cabin altitude and ∆P the same as they were when you began the test. An occasional flow pack is balky to reopen. Give it time. You will know it reopens when the cabin VVI shows a downward surge. ITT will also increase a little and torque will decrease a little.
Once everything is the same as it was initially, switch off the right side’s environmental bleed air and record or film those results. Let’s suppose that this time the peak cabin climb is 600 fpm and the cabin quickly reverses the climb and descends back to the original altitude. Before reading further, take a moment to think about these results and see if you can determine why there is a difference.
Tick-tock-tick-tock-tick-tock. Ok, got your answer?
The answer is that the right flow pack is much weaker than the left. We lost less air when we turned off the right pack and it, when operating alone, was not strong enough to overcome the cabin’s leaks. Yet we lost a lot of air when we terminated the left pack’s flow and it overcame the leaks just fine and was able to maintain full pressurization when operating by itself.
But even one or two strong flow packs may not be able to supply
16 • KING AIR MAGAZINE
JUNE 2018