Page 12 - Volume 12 Number 12

MAINTENANCE TIP
Looking at Pressurization
by Dean Benedict, A&P, AI
True story: I traveled to Texas to check out a King Air 200-300 ft/min; he may not think much of it. A year
at the tail end of its pre-buy inspection. Phases I-IV
had been done and the squawks had been addressed. My job was to put my eyeballs on the airplane, do the final ground runs and go on the acceptance flight. When I asked the pilot to check the cabin leak rate, the Cabin Vertical Speed Indicator (VSI) pegged at 6,000 feet/minute (ft/min). It was a deflating moment (pun intended). Here we were at the eleventh hour of the deal, everyone was anxious to close, and suddenly we had a major snag.
This year alone I’ve encountered several pressurization problems in King Airs at the end stages of the pre-buy. Is this a problem with the shops? With the pilots? I can’t point a finger in one direction. I think it’s a little of both, coupled with a lack of understanding of the pressurization system.
Gradual Deterioration
Typically, a pilot pulls the engines back to 85 per- cent and sees a little uptick in the Cabin VSI, say
10 • KING AIR MAGAZINE
or two later the Cabin VSI reads 500-600 ft/min – not a glaring change. The trend continues but the pilot keeps forgetting to squawk it at the next Phase.
Shops, on the other hand, don’t always do full-blown ground runs to check every system, and oftentimes they can’t test fly the aircraft to check the cabin leak rate or see if it makes max differential. Thus, pressurization sometimes escapes close scrutiny.
Pressurization becomes anemic when the input from the flow packs is too low. Flow pack performance weakens over time. Occasionally you’ll get a sudden fail in a flow pack accompanied with a noticeable drop in the ITT on that side. More usual is a gradual decrease of inflow from one or both packs over several years.
On the other side of the equation, you have leaks that develop over time in the pressure vessel. It has a lot of seals and they don’t last forever. Again, the change is gradual. Low inflow or high outflow? In many King Airs there are problems on both sides.
DECEMBER 2018