Cabin Pressurisation

[a5130]

In regards to the subject matter, one of our F100 aircraft have been experiencing pressurization defects. We have carried out the AMM fault isolation and T/shooting procedures with replacement of the cabin pressure controller, the primary and secondary outflow valve on numerous occasions. We also did the bite test and found satisfactory. After all this, the defect will come on again especially on descent or cruise...appreciate any helpful tip...Thanks.

 

[3DDave]

Did you check to see that no one left a door open? You think I kid, but I was on a flight where daylight and part of the wing were clearly visible through the emergency exit door seal. Mostly it was noticeable due to the engine noise coming through the opening. It did seal up at altitude, but if something got stuck in there it wouldn't.

Also, if you do a ground test for cabin pressure leaks - I've seen pictures of what happens to unattended aircraft hooked to unregulate pressure. Not pretty.

I've guessed pressure too low. Is that the problem? What symptoms suggested looking?

 

[Sparweb]

Did I miss something? Or did you forget to tell us what the defect is?


STF

 

[a5130]

The cabin pressure fluctuates when descending or while on cruise. It sometimes occurs when the engine is power up for take off. In such cases, the crew usually switch off AUTOMATIC and do the manual operation. Sometimes on descent, cabin pressure control fault message and manual procedure is applied. Electrical wiring checked with satisfactory.

 

[Sparweb]

Thank you for filling in some info. I hope you don't mind if I ask for more:
How much does it fluctuate? 100's of feet? 1000's? How rapidly does it change?
Can the problem be re-created on the ground, or does it only occur at high altitude?
Some thoughts:
Blockage in cabin recirculation ducts.
Blockage in vents where cabin air should flow out to reach the outflow valve(s).
Leaks or faults in the bleed air system, or its shut-off valves, or the air conditioning system, such as a flow control valve that is sticking or acting erraticly.
Failed pressure sensor causing the automatic control system to oscillate between two states, but it thinks the state is constant.

Have you contacted Stork about this?


STF

 

[a5130]

Thanks SparWeb,

We have not contacted Stork, however other operators have reported the same issue on their F100 and the defect was related to blocked pressure lines, Cabin Pressure Selector Panel, faulty pneumatic hoses behind the cabin pressure selector and wiring issues that supplies the control signal from the CPC to the Torque Motors of the outflow valves. Our engineers have been trying to create the defect on ground but the system is working fine, cannot duplicate fault. Most reports states that during take off, cabin pressurization descended at over 2000 rpm. At other occasions, after engine start-up, a/c start pressurizing at 700 fpm. We had a long history on this issue on the same aircraft since 2010 and looks like we have been going around in circle in rectifying the defect.

Anyway, thanks for your input above, we will look at those areas again.

 

[a5130]

In addition, I would like to add the following symptoms to the subject defect:

1. Humming noise & vibration above cabin ceiling, sometimes from aircon system, below flight deck floor or aft of capt's sliding door - inspection c/out on vents, ducting, components with nil findings.

2. The fault is experienced only when selected to AUTOMATIC, Manual operation works ok.

 

[Sparweb]

Are these faults associated with the load on the air conditioning system, or the bleed air system? Wing de-ice etc.??
Isn't 2000 fpm the maximum rate? I'm sure the flight crew didn't set it to do that. Does your controller have a rate limit setting? Did the flight crew attempt to reduce the rate before switching to manual?
How about starting up with one AC pack shut off? Not in terms of limiting the abrupt cabin altitude change, but more like isolating a potential source of confusion as you continue to troubleshoot.

I am flattered to hear that my help so far has been better than the customer support you expect from the Dutch, but I still think you need to ask them.


STF

 

[a5130]

Thanks for your reply.

Will have to find out more about your queries and will contact the OEM with the history of the defect...will further update on the outcome.

 

[3DDave]

"Most reports states that during take off, cabin pressurization descended at over 2000 rpm." But the initial problem was described as "After all this, the defect will come on again especially on descent or cruise."

So is it takeoff or descent and cruise? I'm so confused.

Assuming it's losing the equivalent of 2000fpm, does it continue for long periods or is it very short term? Is there any way to monitor the operation of valves to see if they are responding as expected; that the valves are getting signals and acting on those.

Interesting report:

http://www.cabinfiles.com/?CFrequest=file;01005001100126

The report is more about emergency/unexpected pressure loss, not recurring problems, but it does have a table of percentages of failures that might help, from which it reports "Control problem 44.1%," excluding actuators and such.

"In 5.4 per cent of cases, the problem with the pressurisation system was attributed to the outflow valves. Maintaining a constant cabin altitude is a balance between the entry of pressurised air and the outflow of this air. If the outflow valves are not operating properly, cabin altitude will not be maintained at the desired level. Similarly, if there is a leak in the cabin, pressure will be lost. In 14.7 per cent of cases, this leak was due to problems with doors and windows. The leaks were due to several reasons, including faulty door and window seals, cracked windows, or improperly closed doors. In general, these events resulted in inability to maintain the desired cabin altitude, even though the cabin pressurisation system was otherwise working normally. The rate of cabin pressure change in those cases was generally slow, readily identified by the crew, and an uneventful descent was generally carried out.

In the majority of cases in this study where the data was available, the rate of cabin pressure loss was relatively slow. In 59 per cent of cases the rate of cabin pressure change was less than 2,000 fpm, with an average rate of approximately 1,700 fpm. The finding in this study, that most decompressions tend to be relatively slow, is consistent with other studies. Files et al also found that 83 per cent of their series of 1055 depressurisation events involved a slow loss of cabin pressure (8)."