How to depressurize an aircraft

[ottawa]

If the outflow valve is stuck in the closed position, and the aircraft is on the ground and pressurized, what would be the easiest way to depressurized the aircraft from the outside? Opening the cargo compartment vents? Thank you. Attached is the picture of the L1011 Flight SV163 where there was a fire but th eoutlow valve was stuck closed and the occupants could not escape.

[AmarokGTI]

On the type I fly, there is a cockpit hatch that can be opened on the ground for ventilation and also to resolve any pressurization issue up to 0.3PSI.
Aside from that, there is a pressure “dump” switch in the cockpit that drives open the electoneumatic valve. The pressurization system can also be switched manual mode and the secondary outflow valve used. (Much slower response time).

[pattern_is_full]

First - remove the source of the pressurization (the engines' compressors, or in a pressurized piston, an engine-driven pump). Shut down the engines ASAP and/or turn off the bleed air switch. Most planes leak enough to lose their pressure eventually if no new air is being pumped in. "Eventually" can mean 20-30 minutes to leak enough to get the doors open - but the bleed air can be switched off in flight, once below 10000 feet actual altitude, to give the leakage more time - IF one knows the valve is stuck ahead of time.

Second - Use the manual pressure dump controls in the cockpit (often a separate system/mechanism from the automated pressure control system: vacuum, hydraulics, electrics, magnetic solenoids, or muscle power).

Third - Some outflow valves can be accessed - when on the ground - from outside the aircraft. Ground crew can open them to dump the pressure.

Fourth - other means that professionals will know about - not for public consumption.

All of the above will vary depending on aircraft type.

In the case of SV163, one never knows. The crew did not shut down the engines ASAP - taxied for 2 minutes+ after landing and took 3+ minutes after landing to shut down the engines. They may have been overcome before being able to operate the backup pressure relief systems (if they even realized they had stuck outflow valves in addition to a fire). The ground response may not have known how to access the valves from the outside and open them (if the L-1011 valves were even externally accessable - specific model, 38 years ago).

[tonytales]

I posted on this very issue on a different thread a while back. The L1011 used electrically operated actuators on the HP Cock on the engines, the fuel emergency fire shutoff valves, the engine pneumatic bleed valves and the pressurization outflow valves. There were NO mechanically actuated valves in those systems. In the event of the loss of electric power you could not:
Shut down the engines
Shut the engine pneumatic bleed valves
Shut the packs down
Operate the outflow valves
Cabin pressure (assuming the outflow valves were not already full open prior to loss of electric power) would prevent any of the eight passenger doors, cockpit escape hatch, baggage doors, galley service door or external access doors to FESC, MESC, hyd compt, etc. from being opened. That's because they were all inward opening doors. The pax door actuator did not have enough power to overcome the pressurization load. The doors are big, lots of square inches of surface area producing a lot of force holding the doors closed. That's why that planeload of pax perished when the fire killed electric power. No way to open any door from the inside or the outside.

[FlightDetent]

Thank you, sir. What an insight!

[Double Back]

It was a sickening crash, and generated then a lot of questions regarding the flight crew's performance, however as Pattern is indicating, at what time they could not act any more. You don't even want to think about the horrendous scenario for pax and crew the last minutes inside this death trap. And not even flying but "safe" on the ground....

[FE Hoppy]

The engines were shut down and the packs were off.
The emergency release handles on the pax doors were not operated.

Terrible accident that would have been avoided if they had stopped on the runway and evacuated as was mentioned by the captain just before landing.

FE Hoppy 5000hrs L1011 in all it's guises.
You can't blame the aircraft for this one.

[Capt Fathom]

when the fire killed electric power. No way to open any door from the inside or the outside.

Crash Axe through a passenger or door window would do it. Although in this case, they obviously didn't realise the danger they were in.

[tonytales]

Years before, I saw a Crash Crewman swing a full size fire axe at a pax window. It hit, bounced off and throw him on his back on the wing. Windows are tough.
L1011 had a heavier than usual skin due to its construction but still, that would be the only way to release pressure. .
I do see in the report the engines were shutdown but it was six minutes after landing. With the toxic gasses formed by burning cabin material I am sure it was very bad inside. We had a fire in the FESC on an L1011 at KEWR when I was Maintenance Manager. Fortunately it occurred as the aircraft approached the gate. The FESC overboard fan was discharging heavy black smoke and tendrils of burnt insulation. Power did not fail until after engine shutdown and PAX door opening so everyone got off although the last were in a big hurry as, after loss of power, the fumes from the FESC were now coming up to the main cabin. One of my maintenance foreman spent a night in the hospital due to short exposure to the fumes as he ran in, opened the cockpit overhead hatch and ran out again and collapsed in the loading bridge. I had to borrow a breathing pack from the PONYA fire crew to enter the aircraft. The fire had been extinguished by a potable water line bursting from the heat. I suspect six minutes plus the time in the air reduced the capacity of anyone inside to open a door.
We ignited a small sample of the shroud that covered the toilet tank as a test and the fumes drove us out of the maintenance office. This is nit to say the L1011 had worse interior material than others. They are all the same and thy use the same stuff. It did however lack any manual mechanical means of dumping cabin pressure in the absence of electrical power.

[FE Hoppy]

Years before, I saw a Crash Crewman swing a full size fire axe at a pax window. It hit, bounced off and throw him on his back on the wing. Windows are tough.
L1011 had a heavier than usual skin due to its construction but still, that would be the only way to release pressure. .
I do see in the report the engines were shutdown but it was six minutes after landing. With the toxic gasses formed by burning cabin material I am sure it was very bad inside. We had a fire in the FESC on an L1011 at KEWR when I was Maintenance Manager. Fortunately it occurred as the aircraft approached the gate. The FESC overboard fan was discharging heavy black smoke and tendrils of burnt insulation. Power did not fail until after engine shutdown and PAX door opening so everyone got off although the last were in a big hurry as, after loss of power, the fumes from the FESC were now coming up to the main cabin. One of my maintenance foreman spent a night in the hospital due to short exposure to the fumes as he ran in, opened the cockpit overhead hatch and ran out again and collapsed in the loading bridge. I had to borrow a breathing pack from the PONYA fire crew to enter the aircraft. The fire had been extinguished by a potable water line bursting from the heat. I suspect six minutes plus the time in the air reduced the capacity of anyone inside to open a door.
We ignited a small sample of the shroud that covered the toilet tank as a test and the fumes drove us out of the maintenance office. This is nit to say the L1011 had worse interior material than others. They are all the same and thy use the same stuff. It did however lack any manual mechanical means of dumping cabin pressure in the absence of electrical power.

Like pretty much every modern aircraft.
Anyone want to list aircraft with manual dump valves?

[tonytales]

I believe the DC-10 had the same lollypop handle for manually controlling the two outflow valves as the DC-8 and DC-9. Can't speak for the MD-11. The valves were normally electrically actuated and you could see the "lollypop" knob moving as the system operated. In the event, you pulled up on the knob and disengaged the system from the electric actuator and could set the valve openings where desired.

[harrryw]

Don't the fire crews puncture the fuselage with a lance nozle.....I would think this would work if they knew it was neededl

[FE Hoppy]

I believe the DC-10 had the same lollypop handle for manually controlling the two outflow valves as the DC-8 and DC-9. Can't speak for the MD-11. The valves were normally electrically actuated and you could see the "lollypop" knob moving as the system operated. In the event, you pulled up on the knob and disengaged the system from the electric actuator and could set the valve openings where desired.

Yeah, the comet had a hand wheel operated manual valve.
Anything modern?

[dixi188]

A300 has a wheel that opens a manually operated outflow valve.

[A Squared]

Anyone want to list aircraft with manual dump valves?

DC-6 had a "Johnson bar" beside the copilot's seat which would open the outflow valves and de-clutch the pressurization superchargers. C-130 has a cable release for a panel in one of the overhead hatches.

[Goldenrivett]

Originally Posted by FE Hoppy https://www.pprune.org/images/buttons/viewpost.gif (https://www.pprune.org/tech-log/609694-how-depressurize-aircraft.html#post10166350)
Anyone want to list aircraft with manual dump valves?
The Vanguard (VC9 - AKA Whispering Warehouse) had a manual "unpressurised flight valve" on the co-pilot's side.

But they all depend on someone alert enough to operate them.
The Saudi Arabian TriStar only needed the Flight Engineer to put the pressurisation controller back to "Normal" and the squat switch logic would have opened both outflow valves fully. The system was left in the "Standby mode" and so the outflow valve positions were determined by the Flight Engineer. (The Fwd was found Fully closed, the Aft almost fully closed).
See http://www.aviation-accidents.net/report-download.php?id=43 page 81.

Once all electrical power was lost, the valves are held in their position by brakes.

[FE Hoppy]

The Vanguard (VC9 - AKA Whispering Warehouse) had a manual "unpressurised flight valve" on the co-pilot's side.

But they all depend on someone alert enough to operate them.
The Saudi Arabian TriStar only needed the Flight Engineer to put the pressurisation controller back to "Normal" and the squat switch logic would have opened both outflow valves fully. The system was left in the "Standby mode" and so the outflow valve positions were determined by the Flight Engineer. (The Fwd was found Fully closed, the Aft almost fully closed).
See http://www.aviation-accidents.net/report-download.php?id=43 page 81.

Once all electrical power was lost, the valves are held in their position by brakes.

Yep.
In much the same way as he would have had to remember to open a manual valve. But he didn't.

[Pugilistic Animus]

Det cord...

[vapilot2004]

what would be the easiest way to depressurized the aircraft from the outside?

The negative pressure valve is typically just a spring-loaded panel somewhere along or near the belly. The combination types (negative and over-press) are similar and both types are on all current commercial transport aircraft.

I've seen it done and the only force to be overcome aside from the calibrated spring would be the pressure diff x the area, which on a panel that small (particularly at field elevation) is quite manageable and the resultant outflow is not dangerous.

The valve can be held open, if needed. This operation limits any potential damage to that LRU.

[BAengineer]

It all depends how much pressure you have inside the aircraft. The negative px relief valve is about 6x4 inch on something like a 777 - so that is 24 square inches which at around 8psi is almost 200lb sitting on that door. If it is much lower then the engines are probably going to be off and the pressure will leak away fairly rapidly anyway.

[vapilot2004]

Thanks for doing the math, BAe, however, an 8psi diff on the ground seems rather high. That would put the cabin altitude very far below sea level (circa -10,000 feet) even at an airport like Denver (~5,000 MSL).

[CCA]

On the outside of a 747 classic while talking to the engineers in the cockpit after a pressurization check we couldn't open the main entry door even when the cockpit pressure was indicating zero. The butterfly handles wouldn't budge past a certain point, eventually the upper and lower gates opened with a burst of air rushing out.

As said above the negative relief valves are probably the best bet if you don't want to damage the aircraft by punching a hole in it.

Anything other than near zero diff pressure and you better use an axe or considerably larger if you're in a rush.

[BAengineer]

Thanks for doing the math, BAe, however, an 8psi diff on the ground seems rather high. That would put the cabin altitude very far below sea level (circa -10,000 feet) even at an airport like Denver (~5,000 MSL).

Well I thought the scenario was that the system was not working correctly - after all it the system is perfectly serviceable it would have depressurized on landing.

[vapilot2004]

In practice, it would be fairly difficult for the cabin altitude to be driven that low with a flight deck crew that is present and awake during ground operations, although for this thought exercise, anything is possible and theoretically you are quite right, BAe.

[Chu Chu]

You could always start flushing toilets. :hmm:

[BAengineer]

You could always start flushing toilets. :hmm:

Good trick if you can do it..

I refer the Gentleman to the OP - "what would be the easiest way to depressurized the aircraft from the outside?"

[Chu Chu]

Good point -- I guess flushing the toilets won't work. But you could always send someone to empty them . . . https://assets.publishing.service.gov.uk/media/55015570ed915d141b000011/Boeing_737-8JP_WL__LN-DYS_03-15.pdf