Page 24 - March24

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 enough air to overcome massive
22 • KING AIR MAGAZINE
MARCH 2024