737NG no pressurisation after take off

[Andydandy]

On two occasions I have experienced after take off that we had no pressurisation. No air at all comming into the airplane.
All switches in normal positions, bleeds on, packs on, auto. Duct press 40 PSI both sides. No diff press, cabin climbing with airplane.
First time I tried this on the NG a smart trouble shooter told med to momentarily switch on the eng. anti ice. We did that and instantly the air came back and pressurised the airplane and we could continue the flight.
This is a very weird situation. Not described anywhere, you get no warnings at all, no checklist.
But the trick works. I have tried it 3 times now, in 11 years of flying the NG.

Has anyone tried the same?
Anyone got a clue what this is about as I don't understand it?:8

[rudderrudderrat]

Hi,

I don't know about your NG so I'm just guessing. But on L1011s sometimes the Pack Control Valve(s) wouldn't open, and needed an increase in duct pressure to help open it (them).

Do you take off with the packs off? Because conversely, if your Pack Control Valves are being held closed by too much pressure after take off, maybe the operation of the Engine Anti-ice bleeds off some of the excess pressure and then allows the pack valves to open.

[Andydandy]

Packs were on every time I experienced this. Thank you very much for your answer.

[Sciolistes]

Use engine anti-ice as per the current bulletin. Although it refers to a no engine bleed take off.

[Flight Detent]

I see that the SB you refer to is 737-21-1149, but does not apply to our airplanes.

Also, the 'No engine bleed takeoff' procedure has been removed from our FCOM, though the 'No engine bleed approach' remains.

I'm also quite interested in this "momentary selection of engine anti-ice" that seems to cure no pack output after takeoff.

Andydandy...
When takeoff thrust was set for the initial roll, was the aircon still flowing, and at liftoff were you prepressurized?

That is, did the aircon fail in the roll, and did the outflow valve start closing to prepressurize?

I'm thinking if the aircon flow stopped in the roll, the outflow valve would continue to try and pressurize and would finally end up completely shut just after liftoff.
So, when you did your little trick anti-ice selection, and the two packs came right back, you would have experienced an immediate cabin pressure rate of something quite high (very noticable) for a couple of seconds until the outflow valve compensated to control the rate.

I'm thinking if that didn't happen, then the outflow valve must have remained open until the 'switch trick' and then started to close to pressurize.

I really need more info of exactly what happened!

Sounds like a PSEU problem to me!

Cheers...FD...:confused:

[Andydandy]

Hi Flight Detent

Honestly we didn't notice anything untill the flaps were up. You have a relative high sound level during the take off and initial climb, and it was just after the flaps came up I noticed no air was comming into the airplane.