Cessna 340A pressurization question

robertb

robertb

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robertb
I was recently a passenger in a Cessna 340 owned by a buddy of mine. We were at 16,500'. Due to a 6,500' hill between us and the destination, we needed to make a steep descent as we approached and once we passed the hill. When he pulled the power back to a pretty low setting, the cabin pressure dropped at a rather uncomfortable rate. When I looked at the cabin pressure rate of change dial (or whatever it is called), it was pegged on the negative side.

So, my question is: Is this normal? Is there a way to reduce engine power and still have a sufficient amount of high pressure air from the turbos to keep the decreasing cabin pressure comfortable?

Thanks in advance!
 
robertb said:
I was recently a passenger in a Cessna 340 owned by a buddy of mine. We were at 16,500'. Due to a 6,500' hill between us and the destination, we needed to make a steep descent as we approached and once we passed the hill. When he pulled the power back to a pretty low setting, the cabin pressure dropped at a rather uncomfortable rate. When I looked at the cabin pressure rate of change dial (or whatever it is called), it was pegged on the negative side.

So, my question is: Is this normal? Is there a way to reduce engine power and still have a sufficient amount of high pressure air from the turbos to keep the decreasing cabin pressure comfortable?

Thanks in advance!
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Sure there's a way, don't pull the power all the way back. Piston engines, unlike turbines, don't have tons of bleed air for pressurization. They are instead relying on the turbocharger to provide that air, and if the engine is at or near idle, the turbo just isn't turboing enough.

A normal descent works fine because the engine is still spinning fast enough, making the turbo work and produce enough air for pressurization. But if you pull it back to idle, there's just nothing there.

The same thing will happen if you practice stalls while pressurized, the cabin altitude rapidly increases once you pull the power to idle. (I typically will operate unpressurized when doing air work with a student for that reason.)

About the best you can do is to plan your descent carefully, or live with the temporary increase in cabin altitude. If your friend had pushed the power back up just enough to regain pressurization, it may have necessitated making a 360 for the descent, or extended vectors, or something like that, but it would have kept the cabin altitude from going crazy.
 
That makes sense. Thanks! As the discomfort level was impressive, I pointed to the cabin pressure gauge and he did bring power back in a touch which slowed the rate of depressurization to an acceptable level.
For a normal descent (without an obstacle in the way), I would expect to begin coming down about 3 miles per thousand feet needed to keep things consistent and stabilized. He said that he needed to start it about double that because the plane is speedy. That didn't make much sense to me as 3 degrees is 3 degrees no matter how fast you go.

Even for slick and speedy planes, I would think these two rules of thumb still work:

1. 3 miles:1,000' to calculate distance from destination
2. descent rate = ground speed x 5

Thoughts?
 
robertb said:
When I looked at the cabin pressure rate of change dial (or whatever it is called), it was pegged on the negative side.
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Just to clarify, the cabin rate of climb was pegged on the descent side? If so, he passed his pressurization on the way down, and it had nothing to do with power setting.
 
MauleSkinner said:
Just to clarify, the cabin rate of climb was pegged on the descent side? If so, he passed his pressurization on the way down, and it had nothing to do with power setting.
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Yes. I saw the cabin rate of climb pegged on the negative side of the gauge. This occurred during the early part of the descent and when he reduced power to idle, iirc.
 
robertb said:
Yes. I saw the cabin rate of climb pegged on the negative side of the gauge. This occurred during the early part of the descent and when he reduced power to idle, iirc.
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Reducing power would decrease the air coming into the cabin, resulting in a cabin climb if anything. Sounds like he’s got a pressurization system problem.
 
robertb said:
Yes. I saw the cabin rate of climb pegged on the negative side of the gauge. This occurred during the early part of the descent and when he reduced power to idle, iirc.
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Wait, I missed that, or misread it, or read what I wanted to read, or whatever.

If you really saw the cabin rate peg negative, that would only happen if you're completely depressurized, at which time it would read the same as your aircraft's descent rate.

I suppose it's possible that he pulled the power to idle at altitude to start the descent, therefore removing the inflow of air. The cabin then gradually depressurized (cabin altitude increasing/cabin rate shows a climb). Depending on the descent rate, this might be masked by the aircraft descending. At some point the cabin gets to ambient pressure and at that point the cabin rate would match the descent rate.

Makes my head hurt.
 
robertb said:
Is this normal?
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No. More than likely the airframe has excessive leaks or the outflow valves have issues. But even when one of those systems was up to spec, its not the best performing system due to the low-pressure differential.
 
robertb said:
For a normal descent (without an obstacle in the way), I would expect to begin coming down about 3 miles per thousand feet needed to keep things consistent and stabilized. He said that he needed to start it about double that because the plane is speedy. That didn't make much sense to me as 3 degrees is 3 degrees no matter how fast you go.

Even for slick and speedy planes, I would think these two rules of thumb still work:

1. 3 miles:1,000' to calculate distance from destination
2. descent rate = ground speed x 5

Thoughts?
Click to expand...
Yes, 3 deg is 3 deg, but that's not always a good descent rate for a faster piston airplane. If you're cruising at 200 kts, that's 1000 fpm down that you need. That may require a pretty low power setting. I prefer to descend at around 650 fpm in these airplanes just for engine health, and it keeps the turbo spinning and pressurization working too. That does require starting down further out.

I was in a 340 one day doing 300 knots GS. That would have required about 1500 fpm for the descent. No thanks, that would have been at idle power I'm sure. At 650 fpm we had to start down a long way away.
 
MauleSkinner said:
Reducing power would decrease the air coming into the cabin, resulting in a cabin climb if anything. Sounds like he’s got a pressurization system problem.
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My apologies. Perhaps since the needle swung all the way around, it was pegged but I could not see if to which extreme it was pegged.
 
Bell206 said:
No. More than likely the airframe has excessive leaks or the outflow valves have issues. But even when one of those systems was up to spec, its not the best performing system due to the low-pressure differential.
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I can’t imagine trying to keep a 50 year old airframe from not leaking. Dump valves but all the control cable seals and everything else.
 
robertb said:
the rate of depressurization to an acceptable level.
For a normal descent (without an obstacle in the way), I would expect to begin coming down about 3 miles per thousand feet needed to keep things consistent and stabilized. He said that he needed to start it about double that because the plane is speedy. That didn't make much sense to me as 3 degrees is 3 degrees no matter how fast you go.
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Your terminology is unclear. The cabin climbs or it descends. Not sure which you consider "negative".

Normally, the cabin will begin to descend at the top-of-descent, either automatically or when the pressurization controller is reset to landing field elevation. That is accomplished by the controller modulating the outflow valve toward the closed position while the inflow air is maintained. If you have insufficient inflow air, from too-low of a power setting, then the outflow valve will completely close but the cabin will climb.

If you forget to set the landing altitude in the pressurization controller, preventing the controller from starting the cabin altitude descent, you will become unpressurized as the airplane descends below cabin altitude and the cabin rate-of-deescent will match the airplane's rate-of-descent.

The bigger problem, in an airplane such as the C-340, is engine cooling. You want to slowly reduce power, over an extended time period during the descent, so that the engine can cool slowly.
 
I have no problem descending at 1,000fpm (or more if needed) leaving the engines at cruise power settings (31” mp/ 2350rpm) in my 414 which had the same engines as a 340. Never gets the airspeed into the yellow. This keeps the pressurization air flowing and lets the controller lower the pressurization at a reasonable rate without busting eardrums. Keep the power up.
 
I suspect the cabin pressure became the outside pressure because the turbos could not provide enough flow to pressurize. But it seems like a moot point. Once the cabin is at the aircraft's altitude both are coming down at the same rate. Here I digress, Barberpole liked to start TOD in the 72 at a close in point and and barberpole down when he could in the Caribbean. Me, FE, Mel, "I can't keep the cabin up with the airplane". Mel, "I haven't landed without a cabin yet". Sometimes he gave me an inch up from idle on #3.
 
Was this an older (early) airframe? I believe early model Cessna twins had a simpler pressurization control, that didn't 'engage' until 8000 feet, or would be totally 'open' descending through 8000.
 
Larry in TN said:
Your terminology is unclear. The cabin climbs or it descends. Not sure which you consider "negative".
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Sorry. I don't fly pressurized aircraft and my terminology is likely off. But, if the cabin altitude is decreasing, I consider that change moving in the negative or descending direction.
 
Jim Carpenter said:
Was this an older (early) airframe? I believe early model Cessna twins had a simpler pressurization control, that didn't 'engage' until 8000 feet, or would be totally 'open' descending through 8000.
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It is a 1979 340A model.
 
robertb said:
Sorry. I don't fly pressurized aircraft and my terminology is likely off. But, if the cabin altitude is decreasing, I consider that change moving in the negative or descending direction.
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If the cabin altitude is decreasing, the cabin is descending. Sounds like the airplane descended below the current cabin altitude and the airplane was unpressurized.

Either the descent was too fast or the cabin did not start descending at top-of-climb. That could mean a problem with the pressurization controller or that the pilot forgot to reset the landing elevation at top-of-descent. A too fast descent is usually not a problem in a piston because of the need to slowly reduce power over an extended time period. A power-on descent limits your rate of descent and allows time for the cabin to descend at a comfortable rate.

The increase/decrease, positive/negative, terms are confusing because as the cabin altitude increases, the cabin pressure decreases. It's easy to get confused. Use climb or descend and it more clear.

Others have mentioned cabin leaks. All cabins leak. It's not a problem as long as the outflow valve has sufficient authority to compensate for it.
 
Larry in TN said:
The increase/decrease, positive/negative, terms are confusing because as the cabin altitude increases, the cabin pressure decreases. It's easy to get confused. Use climb or descend and it more clear.

Others have mentioned cabin leaks. All cabins leak. It's not a problem as long as the outflow valve has sufficient authority to compensate for it.
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Thanks Larry! I appreciate it.
 
Larry in TN said:
All cabins leak. It's not a problem as long as the outflow valve has sufficient authority to compensate for it.
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At the piston powered level, it also includes the pressurization air source condition especially with those older low differential systems. So you can have new outflow valves but limited pressure air and still have the same issue. Sometimes it can be as simple as if the airframe was "converted" to pressurization vs purpose-built for pressure by the OEM how efficient the overall system is even when new.
 
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