B737 manual outflow valve

[seifly]

Dear all, If anyone can help with a reference manual, If one lands with the outflow valve closed in manual , will the airplane lands pressurised ?

[Capt Quentin McHale]

seifly,

From memory...... the answer is no because the Safety Relief Valves (either side of the outflow valve) will open to prevent cabin overpressure. Not sure at what psid they open at, but best to refer to relevant AMM for actual safety valve ops.

Rgds McHale.

[MechEngr]

If the system limits a pressure above the local atmosphere, which is what a Safety Relief valve normally does, then the plane would remain pressurized. Being as pressure is relative, if it was keeping 5 psig (delta pressure) at altitude it could maintain that on the ground. Apparently cruise is typically 7-8.5 psig with the lower value used to increase allowable cycles and the upper limit to be more comfortable for passengers.

The question then is, will the air back-flow to the engines from the fuselage the same way it came in. There appears to be a valve that will close on low-pressure sensing to prevent back-flow, but I doubt it would automatically close on engine shutdown. If it doesn't the remaining pressure should only make it impossible to open the doors until it bleeds through the typical leak paths passenger jets seem to have. (Like the over wing door seal that I saw a 1/8th gap until they ran the engines up and forced the door out. I noticed it from the amount of engine noise next to my head.)

To add - if one were to seal the plane like a bottle at altitude and not maintain that pressure difference continuously and then return it to the ground there would be more pressure outside than inside - there are relief valves for that. One system my company made for B-52s had that valve fail and the electronics package inside the cylindrical enclosure got shrink-wrapped when the plane returned to the ground.

Example: at 12,000 feet the outside air pressure is roughly 9.3 psia. At 30,000 ft it is roughly 4.3 psia with the cabin at 12,000 ft it would require a difference of 5 psi.

If one were to land with a 9.3 psia in the plane it would experience nearly -5psig and likely beer-can large portions of the fuselage; there are under pressure valves to prevent that. OTOH if one maintained 5 psig to the ground it would be about 19.7 psia inside, but still only 5 psig, just the same as when flying at 30,000 ft.

[Flying Wild]

Dear all, If anyone can help with a reference manual, If one lands with the outflow valve closed in manual , will the airplane lands pressurised ?

Yes, because that is how they do a ground run cabin pressure test.

[Zar_1]

Pls do correct me if I'm wrong, but the safety valve IMO is just there to prevent an overpressure, NOT to enforce depress on landing.

[Jhieminga]

...because the Safety Relief Valves (either side of the outflow valve) will open to prevent cabin overpressure. Not sure at what psid they open at, but best to refer to relevant AMM for actual safety valve ops.
Somewhere between 8.65 and 8.95 psi difference according to my notes.

To add - if one were to seal the plane like a bottle at altitude and not maintain that pressure difference continuously and then return it to the ground there would be more pressure outside than inside - there are relief valves for that.
The negative pressure relief valves open at 0.1 psi difference (more pressure outside than in).

Dear all, If anyone can help with a reference manual, If one lands with the outflow valve closed in manual , will the airplane lands pressurised ?
I can't share a manual, but I think there are many examples available online if you look around. Your question has already been answered, but please note that it is normal to land with a slight overpressure, about 0.15 psi, for structural and comfort reasons (that's what I've read). This would normally be equalised after touchdown. In your scenario, based on the conditions, you would land with a larger pressure difference but it would never be more than the max structural difference and there is also a crew up front who would have been monitoring the cabin pressure during descent. The outflow valve would not be fully closed except in a situation where a structural issue would require closing it fully to prevent or slow down a decompression scenario.

[rayfill]

If you check the QRH for "auto fail" it says after the approach checklist:

At Pattern Altitude:
Outflow VALVE switch . . . . . . . Move to OPEN until
the outflow VALVE
indication shows fully open
to depressurize the airplane

So if you skip this step, you will land pressurised.

[70 Mustang]

“The negative pressure relief valve is a mechanical device and operates independently.It does not interface with other airplane
pressurization systems and requires no crew action.
The negative pressure relief valve is a flapper type valve. The valve hinges on its top edge and opens inward. A spring on its hinge pin holds the valve closed.
Negative differential cabin-to-ambient pressure opens the valve. The valve opens when pressure outside of the airplane is 1.0 psi more than the pressure inside of the airplane (-1.0 psid)

The negative pressure relief valve is on the lower aft fuselage, on the right side, near the aft service door.
Access to the valve is from the aft cargo compartment.

There are two positive pressure relief valves. They are on the lower, aft airplane fuselage. One valve is on each side of the aft outflow valve.
General Description
The pressure relief valves are fail safe devices that bleed fuselage pressure overboard if the aft outflow valve fails closed.
The positive pressure relief valves are mechanical devices and operate independently. They do not interface with other airplane pressurization systems and no crew action is necessary.
The positive pressure relief valves are pneumatically operated by cabin-to-ambient pressure differential. They control pressure to a nominal 8.95 +/- 0.15 psi more than ambient.
When the differential pressure is too high, the valve opens. The open valve lets air out of the airplane. This relieves the cabin pressure. When the cabin-to-ambient pressure is safe, the valve closes.
The positive pressure relief valves have filters. The filters clean the air used in the internal servo and actuator mechanisms.”

I personally seen the positive pressure valves not operate in an old clapped out 200. The differential gauge have swung around past the max to almost the 12 o’clock position. We had had to use manual pressurization mode, but the FO had fallen asleep and had not periodically opened the valve. We were in an intense thunderstorm activity and we had both agreed that I do the radio transmissions for the multiple and complex heading changes in the midst of several lines of thunderstorms. I could not conceive of anyone falling asleep in such a demanding environment, but he did. And I was too busy running the range and angle of the radar so much that I hadn’t noticed his “checking out.” After getting through the thunderstorms, I did a quick scan and saw the pressurization gauge looking like I had never seen it in flight. It looked like it would on the ground. Thankfully the aircraft was able to take an extreme overpressure.