Advise on pressurization

[dominique]

Flying the gorgeous RealAIr T-Duke brings the issue of the rate of (de)pressurization during the climb and descent. 

 

- The T-Duke climbs, in the most favorable conditions, at 4000 f/m at about 130 knots up to FL250. Yes, Sir  !  . What would be the best cabin pressurization rate for the confort of the souls aboard ?

 

- I make it descend at 900 f/m at 180 kts which is a little less than the cruise speed (Vne is 198). Could the descent be swifter ? At which confortable rate should I depressurize the cabin ?

 

Looking forward to your suggestions !

 

 

[spud]

Usually in pressurized A/C the compressor is activated as the A/C passes the alt. the pilot wants the pressurization to hold and as the A/C decends dis-able the cabin pressurization so that the normal atmospheric pressure is in control.  Easier on the passengers that way.  IE: want cabin pressure to be 5000' so start pressurization as your passing that alt. and on decent stop pressurization as you decend through 5000'.

[dominique]

Thanks Spud

 

The Duke cabin controller says it can maintain a cabin altitude of about 10K feet at FL250, so the compressor will kick in at 10 000 feet and maintain that pressure. Should I ask the compressor to follow the 4000f/m rate of climb between 10K and 25K feet ? Bearable for the passengers ? I suppose not...

 

When going down, after 10 000 feet, the outside air pressure will increase rapidly and the depressurization will begin. What would be the confortable rate, like the one used in commercial flight and should I match exactly the aircraft descent rate and the depressurization rate ? Is the idea to reach the equilibrium between the cabin and the outside only at landing ?

 

Sorry for the naive questions, but its rather confusing for me. I've, for years, only simmed with non pressurized aircraft or with simulated oxygen (P51)  

[AKar]

Uh...not really. Nothing like that. There is no "compressor" to kick in generally, but an outflow valve that lets air out from the cabin. It is the primary controlled part of the pressurization system. The bleeds from the engines, whenever they are enabled, push air through the air conditioning pack(s) into the cabin, and the cabin pressure is maintained at correct level by regulating the outflow. In most airplanes the cabin is 'scheduled' automatically during normal operation, the typical inputs being the expected final cruise and landing altitudes. Some small or old planes require a separate cabin vertical speed selection to be done by the crew, which control the rate the cabin pressure is allowed to fall during the climb. I don't know what are the figures generally, but some 300-500 fpm or so should do it in most airplanes.

 

The air conditioning system is typically enabled already on the ground, or right after the take-off, depending on a few things. When the airplane starts to climb, the cabin pressure controller drives the outflow valve to smoothly "rise" the cabin to the preset level, like 8000 ft for example, either at constant vertical speed set by the pilot, or by appropriate rate determined by the final cruise altitude setting.

 

It is kind a bottle to which air is blown in at some rate we don't care much, and the proper pressure inside it is controlled by allowing an appropriate amount of air flow out. The cabin pressure controller does that.

 

-Esa

[dominique]

OK thank you very much Esa, it indeed reflects what the manual says but the Duke manual is still a little short and supposes too much knowledge from the user (at least me) !

 

 

So I set the cabin controller (gauge in the middle) for the planned cruising altitude (or the equivalent in cabin altitude, this is a dual dial) or for the destination altitude +500.

 

I tune the gauge on the left between a rate of 300/500 fpm (which is not much as the gauge is graded in thousands) to avoid ear popping ?   

 

Now the rate of pressurization or depressurisation being much smaller than the actual rate of climb/descent of the aircraft, I've to climb or descent the aircraft to stay in the green of rightmost gauge which  shows the 4.7 psi maximum differential allowed  between inside and outside ?

 

hmmm, I think I get it (or not )

[AKar]

OK thank you very much Esa, it indeed reflects what the manual says but the Duke manual is still a little short and supposes too much knowledge from the user (at least me) !

 

No problem - that's all too often the issue in many addons: the system description are left out for most part even in complex airplanes, that can have (often excellent as far as I've heard) familiarization courses where new pilots go through these things in detail.

 

But yes, the idea of the cabin controller selector is precisely that: at some point during your flight preparations, you dial in your expected cruise altitude. Let us say it will be very reasonable FL240 (above that often takes you into RVSM airspace at least around here - I don't think Duke is equipped and certified for that!). So, you dial in that 24 into the controller.

 

The reason you have to do that selection is that these cabins are 'proofed' only to only some maximum pressure differential. You cannot keep the cabin at sea level when flying at that FL240 for instance. This selection actually sets the final cabin altitude, some 9600 ft, but because you would need charts to check the correct one for each altitude, the selector is indexed for flight altitudes too. The corresponding cabin altitude presets the final pressure differential to a value appropriate to the aircraft model.

 

As you start climbing, the controller modulates the cabin pressure to decrease at the rate you've set towards that final cabin altitude by regulating the outflow from the cabin. Note that if you climb at very fast rate and have set a low rate into the controller, the maximum cabin pressure differential can be reached before the airplane is at the cruise altitude! If that happens, the cabin 'climbs' much faster than you've set because the system won't go beyond the maximum differential, and if it malfunctions and attempts to do so, the safety valve will open and prevent further pressurization. It can't ever go beyond the maximum differential, so you shouldn't worry about that. If in reality one feels in his/her ears that now were climbing faster than we should, a peek to the cabin pressure gauges should reveal the reason: if the differential shows the maximum allowed, you'll know the system is now keeping the cabin at that differential, and it has to climb with us.

 

And yes, it makes sense to set the cabin altitude according to destination should that elevation be above our cabin altitude. It makes a little sense to pressurize our cabin to very low altitude if we cruise at 12000 ft for example, and then violently dump the pressure when landing at, say, 7000 ft! In that kind of case, you'd want to set the cabin altitude to something like you said, 7500 ft, even if the pressure vessel was able to keep a higher differential at your cruise altitude.

 

Note that the names of these system components vary between airplanes - I can't recall how they were called in Duke!

[Rob Newman]

OK thank you very much Esa, it indeed reflects what the manual says but the Duke manual is still a little short and supposes too much knowledge from the user (at least me) !

 

 

So I set the cabin controller (gauge in the middle) for the planned cruising altitude (or the equivalent in cabin altitude, this is a dual dial) or for the destination altitude +500.

 

I tune the gauge on the left between a rate of 300/500 fpm (which is not much as the gauge is graded in thousands) to avoid ear popping ?   

 

Now the rate of pressurization or depressurisation being much smaller than the actual rate of climb/descent of the aircraft, I've to climb or descent the aircraft to stay in the green of rightmost gauge which  shows the 4.7 psi maximum differential allowed  between inside and outside ?

 

hmmm, I think I get it (or not )

 

Thats's how I would do it... At least in the Duke

 

Indeed those controls are pretty standard on a lot of older generation aircraft, including king airs, and BE-1900s... The real key to operation is keeping yourself within that max differential range...

 

Say, for example, you climb at 4000ft/min and climb the cabin at 500fpm... in 5 minutes you're at 20,000ft but your cabin is only at 2,500ft... The air pressure outside your aircraft is approximately 6.75psi (the standard atmosphere at 20,000ft) The air pressure inside your aircraft is 13.4psi:

 

13.4 - 6.75 = 6.65

 

So you are exceeding your maximum differential. What happens in this situation is the "poppet valve" (or sometimes called emergency valve) will trigger once you get to your max differential, so as not to damage the aircraft. What that means to passengers on board however is the cabin altitude will suddenly and rapidly increase, leading to some very nasy ear popping and upset passengers

 

Your goal should be to reach your target altitude (20,000ft) while still staying within your max differential... Your cabin altitude at 20,000ft will be around 6000ft... So your target is to use whatever ft/min cabin climb will achieve that.

 

In the example we used, if it takes 5 minutes to reach 20,000ft (4000ft/min), then you have 5 minutes to reach your target cabin altitude (6000ft). If you reach the cabin altitude sooner, that's good.

 

So in this case, 1000ft/min is still a bit slow (you'll only be at 5000ft in 5 minutes)... 1200ft/min would be right on schedule, but why risk it? 1500ft/min would ensure the cabin altitude is where it needs to be without exceeding max differential.

 

 

This is another reason why you always add the 500ft margin to your cruise altitude also... Planning 20,000ft? set the pressuriasation to 20,500ft... This has the effect of increasing the programmed cabin altitude very slightly, so when you get to 20,000ft, your cabin altitude is actually slightly higher than 6000ft... you are reducing your actual differential and adding a safety margin.

[Rob Newman]

For the mathematically inclined:

 

VRc = 200 + ( Pca / ( Pa / VRa ) )

 

VRc - Vertical Rate (Cabin)

Pca - Planned Cabin Altitude

Pa - Planned Cruise Altitude

VRa - Vertical Rate (Aircraft)

 

 

Example:

 

VRc = 200 + ( 6000 / (20,000 / 4000) )

 

VRc = 200 + ( 6000 / 5 )

 

VRc = 200 + ( 1200 )

 

VRc = 1400fpm

[AKar]

Good additions above. It should be noted that Duke apparently doesn't represent typical small turbine airplane performance characteristics that, say, King Airs and Piper Cheyennes do. Typically even 2000 fpm climb is plenty for a cruise climb, reduced to 1500 fpm or so somewhere near 15 to 20 k. These rough figures I recall from Cheyenne 400, which is not that shabby in performance on its own! Obviously it might do better if one insists on V_Y climb, however, little reason to hang from the props there either.

 

Not sure how the Beech controller works, some I know better keep the maximum differential via regular outflow valve - the safety valve is set to open slightly beyond the maximum (selectable) cabin pressure differential. Neither of the valves, when operating, does 'dump' the pressure should the maximum differential be reached but keeps it at around that - the ear pop effect is proportional to the rate of climb then present.

 

To bring another kind of airplane into the discussion, let's consider Piper Malibu. In there the cabin will not start pressurizing before the airplane passes the set cabin altitude. Such a system is operated by setting it to takeoff field altitude + 500 ft typically to begin pressurizing the cabin at that altitude. It will then maintain a constant cabin altitude until maximum differential is reached, and thereafter it will continue to climb at that constant differential. If one is to climb high enough for that to happen, the pressure control needs to be reset to the cruise altitude setting after the cabin starts to pressurize, after which it climbs to that cabin altitude at the rate of climb set by the appropriate knob.

 

To be honest, I'm not sure whether the system in the Duke should be operated similarly to the Malibu's, or does it start to pressurize the plane as soon as it is airborne (is it having a ground logic) similar to the later airplanes. I assume it is not that advanced that it pre-pressurizes the airplane already during takeoff like one in, say, PC-12 does! Too bad I've got no copy of Duke's AMM or anything.

[dominique]

Good reading that I find for my breakfast, very much appreciated, gentlemen !

[moonman]

A very interesting topic and some very detailed information!

Thank You!