Page 14 - Volume 12 Number 12

In ow (Packs) versus Out ow (Leaks)
If your flow packs are working as advertised, chances are you won’t notice a leaky cabin unless you test for it. Electronic flow packs are especially good at masking cabin leaks. The question you should be asking is: Can each of my flow packs do the job by itself?
Imagine losing an engine at altitude. The flow pack on that side is no longer putting any air into the cabin, leaving the other flow pack to do the job alone. What if it’s weak and you didn’t notice? Worse yet, what if your leak rate is 6,000 ft/min? Current altitude plus 6,000 feet in one minute. Now you’re scrambling for oxygen masks while descending to a lower altitude.
I once squawked pressurization during a pre-buy inspection and the broker for the seller insisted it was not an airworthiness item. He was adamant that his client was not responsible for fixing it. I gave him the above example and he eventually changed his tune.
Leaks – To Find and to Fix
All airplanes leak a little, some leak a lot. Seals go bad over time. Drill bits puncture lines by mistake. Gaps develop. The older the airframe, the leakier it becomes. But if your flow packs are working and the aircraft makes max differential on each flow pack, independently, then cabin leaks are less of a problem.
Remember the Cabin VSI that was pegged? The actual leak rate was somewhere above 6,000 ft/min and those packs tested good. Clearly the aircraft was leaking like a sieve. What happened next, however, blew my mind. The shop brought out a vacuum cleaner to “pump up” the airplane and look for leaks. I’m not kidding; I wish I were!
The only way to address cabin leaks is with a huffer, and not just any huffer. You need one with at least 9 psi capability. Huffers that work off the shop’s air compressor cannot pump the cabin up to max differential. To find and fix cabin leaks without a proper huffer is a waste of time and money. Control cable seals are common, but beyond those it’s a total crapshoot.
I tried three different shops before I found one with a proper huffer for that leaky King Air. As soon as it was pumped up, we found a major leak at the emergency exit. The control cable seals were also bad. Then we found and fixed a number of other leaks that were not too hard to access. We got that leak rate down to 2,800 ft/min. A second acceptance flight was performed and the deal closed soon after. The new owner is thrilled with his King Air.
The factory puts out brand-new King Airs with a leak rate of 2,500 ft/min. On an older airplane I’m happy with 3,000-3,500 ft/min. You can’t make an old airplane new again, but you can make
12 • KING AIR MAGAZINE
DECEMBER 2018