Looking at Pressurization

Looking at Pressurization

Looking at Pressurization

True story: I traveled to Texas to check out a King Air 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 per­cent and sees a little uptick in the Cabin VSI, say 200-300 ft/min; he may not think much of it. A year 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.

The Flow Pack and the Pneumostat

Near the end of another pre-buy inspection, I found a weak flow pack on a King Air and had the same situation – everyone was itching to close the deal and move on. The seller protested vigorously. He said that pack had been changed earlier that year. So, I asked if the pneumostat was changed at the same time and the answer was no. Hmmm.

The flow pack and the pneumostat work together. The pack provides the flow and the pneumostat adjusts the flow rate according to temperature and pressure. The flow pack has orifices that become clogged over time and the pneumostat has bellows that get dirty, decreasing effectiveness. There is no way to test the pneumostat separately unless you have a pneumatic diagnostic bench lying around. In this example, the flow pack was changed and the problem seemed to go away for a while, then it came back because the pneumostat wasn’t up to par.

In all my days of working on King Airs, I learned early to exchange or repair the flow pack and pneumostat together, every time. What it saves in time and aggravation is well worth the few hundred bucks spent on an exchange pneumostat. This is a key point with pneumatic flow packs and I cannot stress this strongly enough.

When I had my shop, I knew a pneumatic repair facility that did fantastic work. I sent flow packs with pneumostats to him all the time. He would repair or overhaul them as necessary. I could get exchange units from him if my customer was in a big hurry. He was able to “marry” the pack and the pneumostat, fine tuning them together – a great bonus.

Electronic Flow Packs

Electronic flow packs offer peak performance over a much longer period of time than their pneumatic counterparts. The down side is they are very expensive to exchange and they are only available from Beechcraft (Textron Aviation).

Electronic flow packs have a thermistor instead of a pneumostat, and they have a controller (not the pressurization controller in the pedestal). If your King Air has electronic packs and you find a weak one, there are a couple things your shop can try before exchanging the pack. The thermistor can be ohm’ d out to ensure it is working properly. To troubleshoot the controller, swap it to the other side. There is also a test box available that checks proper operation of the electronic flow pack system.

Pneumatic packs can be changed to electronic; Beech has a kit.

Inflow (Packs) versus Outflow (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 substantial improvement in the leak rate. Just use a huffer, not a Hoover®.

Do’s and Don’ts

Do check cabin leak rate and your flow packs. To test flow packs (on the ground or in the air) operate them one at a time while observing the Cabin VSI. Start with both packs going. To test the left side, turn the right pack off and watch the climb rate – it should go up briefly as the left pack adjusts to the change of flow, then it should come back down. Repeat on the other side. If the cabin VSI goes up to 500 ft/min that pack is getting weak; 1,000 ft/min indicates a dead pack.

Do check your leak rate too. An excessive cabin leak rate can throw off the results of your flow pack test. Inflow versus outflow – there are two sides to the equation and both must be examined.

Don’t ignore a weak flow pack just because you can make max differential on both packs. A strong flow pack can, and should, handle the load by itself.

Do send the pneumostat with the flow pack whenever repairing or exchanging.

Don’t assume that a pressurization problem is caused by the pressurization controller or the outflow valve. Many pilots point to these before testing their flow packs or checking their cabin leak rate. Yes, controllers and valves can fail, but in a King Air the packs and/or leaks are the primary suspects when pressurization presents a problem.

It’s easy to take pressurization for granted as it doesn’t present a glaring red flag very often. But the system does degrade over time and it merits attention and proper maintenance. Keep an eye on it.

I confess I get great satisfaction from seeing older King Airs with many thousands of hours on the airframe still performing well. I had the great honor of speaking at the King Air Gathering III at Fredericksburg, Texas, in September. Thirty-plus King Airs clustered on the ramp was a stirring sight to see. Chatting and comparing notes with the owners and operators was an even greater pleasure. I hope everyone enjoys the heck out of their King Air.

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