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Roger Mazengarb

Malibu Pressure Update (Part II)

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Alex and Roger et al,I decided to continue with a new thread, as the original was getting a little out of hand. Here's the latest on my test with Alex's fix for the pressurization system of the Malibu:1. The aircraft will now pressurize properly, pressurizing at the selected rate and maintaing the selected cabin altitude once reached. Panel presurization bypass knob on the panel is in (default) - Dump switch on overhead set to pressurize (default).2. If the Dump switch is depressed, she instantly depressurizes - turn it back on and she repressurizes.3. Pulling the Pressure bypass knob when climbing has no effect - the cabin continues to pressurize. Pulling it when level and at selected cabin altitude also has no effect. BUT...when you pull it after setting a lower cabin altitude and beginning a descent, the rate instantly pops to "0" and the pressure goes to ambient. Push it back in during the descent and the plane continues to pressurize and hold rate at the presets. I'd say, leave the thing in and leave it alone.4. Here's something interesting - I decided to test the auto sequences the plane might run when pressurized at too high a level when landing. As I understand it, the normal sequence would be to set the cabin altitude to 500' above our destination field when descending for the approach. For fun, ;-) I set it to several thousand feet above field altitude to see what would happen when we touched down. As soon as I descended below the cabin setting, the pressure began to drop and match actual altitude, ending up at "0" at touchdown (the field was near sea level).

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G'day Randall,>3. Pulling the Pressure bypass knob when climbing has no>effect - the cabin continues to pressurize. Pulling it when>level and at selected cabin altitude also has no effect.>BUT...when you pull it after setting a lower cabin altitude>and beginning a descent, the rate instantly pops to "0">and the pressure goes to ambient. Push it back in during the>descent and the plane continues to pressurize and hold rate at>the presets. I'd say, leave the thing in and leave it alone.I totally agree - leave it IN. The problem can't be huge as pulling the knob works whislt descending (what you said there is correct). It's a neat trick - maintaining pressurisation on ascent without an air supply :-)>4. Here's something interesting - >;-) I set it to several thousand feet above field altitude to>see what would happen when we touched down. As soon as I>descended below the cabin setting, the pressure began to drop>and match actual altitude, ending up at "0" at touchdown (the>field was near sea level).Perfect mate!! The way it should be. :-) This is the way I fly the Hawker all the time. Only no need to exagerate with several thousand feet. just set cabin altitude to field altitude and set up the rate of change and forget. When the aircraft intersects the cabin profile the cabin will then follow the aircraft. You can do the same with the Malibu. Don't have to depressurise prior to landing. ( not sure what the pilots manual has to say on the subject :-) )See attachment.What you describe is normal operaton of a pressurisation system.It is physically impossible for the cabin to have a higher altitude (lower pressure) than ambient whilst under NORMAL operation.you cannot descend the aircraft altitude below the cabin.As you have just confirmed. :-)you cannot climb the cabin altitude above the aircraft altitude.What this means is that if you take off with cabin rate of change SELECTED at 1500 ft/min but only fly the aircraft at 600 ft/min then the cabin will climb at the same rate as the aircraft. (ie 600 ft/min). In effect at this point in time you are flying unpressurised.HOWEVER, it is possible to take the aircraft altitude BELOW cabin altitude. (over zealous piloting or insufficient air supply at idle power setting.) This brings NEGATIVE pressure differential into play. Aircraft are not designed to take the crushing load of negative pressure differential (submarines are :-) ) and so, to protect the aircraft structure, a NEGATIVE pressure relief valve function is incorporated into the outflow valve.This is in effect what happened when you pulled the knob on descent. You cut off the air supply to the cabin, ( you are no longer pressurised) the differential goes negative (ambient > cabin); negative relief valve opens and allows ambient to flood into the cabin. Thus the cabin then descends at the same rate as the aircraft.Sorry for the long winded reply but what we are talking about isn't exactly "eye candy". You can't see all this taking place - it just happens.Beautiful. :-jumpyCheers,Roger @YSSY

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Thanks for all the charts, Roger! Actually, I believe its those "behind the scenes" things (not the eye candy) that make Fly! so great! Not everything has been nailed, but it's gratifying when most items work the way they're supposed to.My hat goes off to Alex for fixing something that's been bugging me since Fly!II came out.I say, upload it Alex! :7

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G'day Alenjandro,Great news ! :-jumpyThanks for the Malibu pressurisation fix.Cheers,Roger @YSSY

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Hello Randall,Your thread reminds me of the last time I flew in a Pilatus PC12. The pilot and I had gone through the printed checklists prior to takeoff but relied on memory during climbout. At 12,000 feet the pilot noticed that while we had selected cabin altitude before takeoff we had not activated it. I had noticed my ears popping but no mental click as to why. He noticed and pressed the button and WOW!!! Instant pressurization can be quite a shock to the system, he quickly set the rate to a lower value and we continued to 20,000 without incident. The guests in back all had a few comments as to what the heck we were thinking.Thanks to you for pointing out the problem with the Malibu and to Alejandro for fixing it.Have fun flyin,Zane

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Hello Zane,Well, that was an interesting story - let me make a guess here...If you were going to cruise at FL 200, then you would probably have selected a cabin altitude from 5,000 to 6,000? If you noticed this at 12,000 and activated it, wouldn't a selected rate still have controlled the pressurization? Or...was the rate not selected, so you had an instant cabin altitude change of around 6,000'? Ouch!!

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I didn't see what the rate knob was set at. Just when the valves were closed and pressure started coming in it was really a surprising feeling, kind of a boom! The pilot reset the rate to around 175 fpm and it was quite comfortable after that point.I'll probably check for that myself next time.Zane

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G'day Randall,I hope Zane went out and bought a lottery ticket. :-) I wonder if he realises just how close he came to doing a "Payne Stewart" (remember the Learjet). We have had that happen to King Air here in Aus. Pure luck that the aircraft rate of change was enough to be uncomfortable to the ears and draw the pilots attention to the problem. Whatever happened to the "Cabin Press." warning?. I checked the ROTW Pilatus (not a definitive reference accepted) and the "Cabin pressure" light comes on at 10,500 ft.BUT BUT ......... The Master Caution lights don't flash!!!!! (for a red annunciator warning???? - madness.If ROTW have got it right then Pilatus have definitely got it wrong. After the surge the cabin would then proceed to it's selected altitude at the selected rate. The rate is always selected (it's only a needle valve)- you can't have a rate not selected: the range is normally about 50 - 2000 ft/min.This post of Zane's is a good example of why to leave the dump vale closed (let the MLG squat switch handle it). If you fail to close it in flight - it could be your last. :-)Cheers,Roger @YSSY

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G'day to you, Roger,Yep, I thought they were treading dangerous water there - the Pilatus is from TRI, not ROTW. I wonder if TRI actually got it right, considering that Zane didn't mention any warnings?

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The Pilatus would have had an annunciator flash and aural warning probably within a thousand feet of the pilot noticing the problem. It's definitely one of those NEVER AGAIN's, if I'm in a pressurized aircraft and notice the pressure changes again I'll certainly check that the switches are set properly.Zane

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G'day Randall,Many apologies to TRI and ROTW.It's just the early onset of dementure. :-lolCheers,Roger @YSSY

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G'day Zane,That helps explain a lot. I was wondering what happened to the warning system.Could you let me know what exactly the ECS switch does to the pressurisation system? I'm just curious.Cheers,Roger @YSSY

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From Pilatus website info on the PC-12:Environmental Control System

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G'day Zeir,Thanks for that. So the ECS switch will probably open a constant mass flow control valve supplying bleed air to the air cycle machine which supplies conditioned air into the cabin. If this is the switch you were referring to as being switched on at 12000 ft then I can well understand the pressure bump you referred to. Air cycle machines spin at unbelievably high rpm's and supply a huge airflow.Thanks for your personal experience story; it certainly added an interesting aspect to this thread.Cheers,Roger @YSSY

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