View Full Version : B737 NG "Pack Trip Off"

Flight Detent
14th Apr 2004, 23:24
Hi all,
I know this has been mentioned B4, but I can't locate the post to find out the response.

My question is:

In the B737 NG QRH, the checklist "Pack trip Off" talks about:

"At level off; Airspeed...290 knots Minimum", with the note saying that the flight deck and cabin temperatures may increase rapidly below 290 knots.

This is with both packs off, flying at 10,000 feet, with the cabin altitude increasing, and the outflow valve presumably closed.

I can't work out how/why the internal temperature of the aircraft is affected by the cruise speed, with both packs off (rapidly).

Can anyone help with a reasonable, short explaination?


15th Apr 2004, 06:54
The minimum of 290 kts is "the minimum airspeed required to maintain appropriate
cabin pressure differential with the outflow valve in the full open position" according to our Flight Manual.

Now regarding the Pack output temps, the following note is in our Flight Manual:

Pack valves may have failed in full open (hot) position.
Note: Flight deck and cabin temperatures may increase rapidly
below 290 knots depending on pack valve position.

This is under both the B737-700 and the B737-800/900. Now, I ass-u-ming that they meant to say Mix Valve failing full HOT or maybe Temp Ctrl Valve (-8/900) failing open. Maybe these would cause a Pack Trip and then the 290 kts minimum is to get enough Ram Air across the Heat Exchangers to cool things down. Otehrwise, I can't quite figure out why they referenced the Pack Valves. These Flight Manuals can be so cryptic sometimes....or maybe it is very simple - but I'm just tired!

15th Apr 2004, 16:57
'The minimum of 290 kts is "the minimum airspeed required to maintain appropriate cabin pressure differential with the outflow valve in the full open position" according to our Flight Manual.'---another mistery
This does not appear in our books,and I think it's definetly wrong.
With 'Pack trip Off" the pack valve is closed,so no matter how much air is going through the ram door.Also the Mix valves,and all valves in the ACM can be in any position,no matter what.As I said,the pack valve,which is before the pack will be closed.
There is a difference between mix valves fail in HOT position (It happed to me once--very hot day on the cocpikt,no matter the speed) and the pack trip off closed.It nthe second case the pack is closed,not working.
The answer to your question is,probably ,that by keeping more than 290 kts,the plane will be ventilated at an increased rate(due to depression created on different areas on the fusselage,that creates suction,and removes hot air from inside.Why hot air? Because of all the electrical wires and instruments,I guess)
So,I guess,It's a question of aerodinamics rather than valves or packes.The air will enter through some valve and exit by another due to pressures created moving fast.
You won't be able to mantain any cabin pressure differential with the valve open,no matter what speed you keep.If anybody knows different ,I stand corrected,but then,there would've been no emergency descent,right? Just increase speed,and it's ok.Nonsense


15th Apr 2004, 17:31
Here is an excerpt from the QRH Procedure for a Pack Trip OFF:

IF Both PACK Lights Remain Illuminated:
Note: Both pack valves may have closed resulting in a gradual loss of
cabin pressure and an eventual CABIN ALTITUDE warning.
Monitor cabin altitude during the remainder of the flight.

IF Cabin Altitude Increases:
Monitor cabin altitude and rate. Descend to lowest safe altitude or
10,000 feet, whichever is higher.

At level off:
Airspeed.............. 290 KNOTS MINIMUM
290 knots is the minimum airspeed required to maintain appropriate
cabin pressure differential with the outflow valve in the full open

Pack valves may have failed in full open (hot) position.
Note: Flight deck and cabin temperatures may increase rapidly
below 290 knots depending on pack valve position.

Pressurization Mode Selector ...... MAN
Outflow Valve Switch ...................FULL OPEN
Right Recirculation Fan Switch .....................................................AUTO
Left Recirculation Fan Switch (If Installed)....................................OFF

IF Flight Deck And Cabin Temperatures Are Excessively Warm:
Flight Deck Door ............................OPEN
Galley or Cab / Util .........................OFF
BCD IFE / Pass Seat (If Installed) ........................................OFF
In-Flight Entertainment Systems..................................................... .OFF
Cabin Lighting ................................DIM
Cabin Window .......................CLOSED

15th Apr 2004, 19:38
FROM Boeing QRH 737 6 June 2003:

If both pack trip off.....

At level off:
AIRSPEED ..........290 MINIMUM
[ Flight deck and cabin temp may increase rapidly below 290 kts]

OUTFLOW VALVE SWITCH...............................FULL OPEN
[Increases airplane ventilation]

IF Flight Deck And Cabin Temperatures Are Excessively Warm:
Flight Deck Door ............................OPEN
CABIN LIGHTING.............................DIM
WINDOW SHADES...........................CLOSED
GALLEY POWER ...............................OFF

As you can see,no mention regarding pressure differential ,or pack valves in hot position.My guess it's,if we're not talking about different planes,then you have some old qrh.Sorry if i'm wrong,no disrespect intended.They must have corrected a wrong information.
Brgds Alex

16th Apr 2004, 06:08
At 290 knots, the airflow flowing around that full open outflow valve is sure to create a low pressure which will help ventilate the cabin. You've still got a recirc fan as well.

Am I missing something here?

17th Apr 2004, 05:29
I think we are missing something.

The question needs be asked. Where does the fresh air enter the cabin?

If you have relatively low pressure just outside the rear outflow valve (remember there is a valve at the front as well, plus an exhaust port), the air will flow out through the valve not in through the valve. This flow will cease when the pressure differential equalizes (that is, the local pressure on either side of the rear outflow valve.)

For the pressure not to equalize, and therefore allowing the flow of air to continue from high pressure (inside the cabin) to low pressure (outside the cabin) through the outflow valve, there must be air flowing into the cabin to replace the outgoing air.

Just like sucking the end of a bottle. The pressure differential is there (between the inside of the bottle and the ambient atmosphere), but because there is no incoming air to replace the small amount of air that has been sucked out, the pressure inside the bottle and the pressure just outside the bottle opening equalizes (the lower pressure inside your mouth), and the airflow stops.

Put another hole in the bottle (preferably at the other end) and the air starts flowing through it.

The aircraft needs that other hole too.

Could it be the forward outflow valve? Afterall it automatically opens when airbourne with low differential pressure. Perhaps there is an airflow reversal through that valve.

[Edited with some extra brackets added to make it clearer.]:)

Flight Detent
17th Apr 2004, 12:25
Hi guys, thanks for all your suggestions,

BLIMP: that forward outflow valve is only opened by two things, one, either aircon pack selected to high, part of the increased ventilation for contaminated air removal, (and then only partially open) and two, with less than 2.5 psi cabin differential,
good thinking, but not the answer, I think.

B73567..., some B737s have two recirc fans, some one, and some none!!

All good, well thought out answers , but I don't think we've found the right one yet.


17th Apr 2004, 17:13
less than 2.5 psi cabin differential,

yes FD ,you got it right.Why do you think it's not the answer?
And B7356.. the airflow at less than 290 kts will probably not be enaugh to create the necessary low pressure.That's what the QRH says. The recirc fan has no influence here,just look at a diagram of the fan airflow design,it just recirculates air,as it's name says.So hot air,even hotter.

18th Apr 2004, 05:10
Actually guys (male and female) after re-reading my manual I think we aren't quite getting it right with regard to what valves are where, and when they do what!!

There is:

Main Outflow Valve at the rear, underneath.

Forward Outflow Valve at the front, side (LHS).

Exhaust Port at the front, underneath.

The forward outflow valve closes automatically to assist in maintaining cabin pressure when the main outflow valve is almost closed or when the recirculation fan (right recirculation fan on B737-400) is operating.

The exhaust port (also known as the Flow Control Valve!) opens to exhaust the cooling air from the E & E compartment overboard during ground operation, unpressurised flight and pressurised flight below a cabin differential pressure of 2.5 psi.
When the "flow control valve" closes, air is directed around the forward cargo compartment liner for inflight heating.

So it seems we are all a little confused. The forward outflow valve position has nothing to do with cabin pressure differential, and all to do with the position of the main outflow valve or recirc fan. I suspect the connection with the recirc fan has to do with smoke removal and the like (as Flight Detent has already mentioned). That is, if you turn the recirc fan off, you are obviously trying to refresh the cabin air as quickly as you can. Opening the forward outflow valve will assist this. This is also assuming the main outflow valve isn't "almost closed".

Also the connection between the Forward Outflow Valve and the selection of High Flow is an indirect one through the Recirc Fans. The Recirc Fan(s) may or may not be operating, depending on whether or not one or both packs are operating in HIGH or not, and depending on which B737 variant you are flying.

The Exhaust Port is the one that opens and closes, depending on the 2.5 psi cabin pressure differential. It's also called the Flow Control Valve because it controls where the flow of warm air leaving the E & E compartment travels.

The Exhaust Port is either OPEN, allowing the air to leave the aircraft immediately, or it is CLOSED, which forces the air to pass around the Forward Cargo Compartment Liner for inflight heating (so the pet pooches and pussycats don't freeze their little paws).

I think from memory, the Exhaust Port is angled at about 45 degree to the aircraft skin, and is pointing rearwards, making the hole an oval shape.


Now back to the original question.

[Edited because I said something I shouldn't have.] :rolleyes:

18th Apr 2004, 05:46
Very interesting question...
And I did not know that some B737s did not come with Recirculation Fans.
I know that the B737-3/5/6/700s have ONE and the B737-4/8/900 have two.

And regarding the Forward Outflow Valves, they only discharge air overboard if the Recirc Fans are off and the Main Outlfow Valve is open. It is the Flow Ctrl Valve that works off of Differential pressure on the Classics.

19th Apr 2004, 08:44
The NG B737 does not have a Forward Outflow Valve,only the classic is fitted with a fwd outflow valve!

19th Apr 2004, 15:25
:O oops!

Thanks for that DDG. I did grab the Classic manual by mistake. Sorry about that everyone.

I do still wonder where the fresh air enters the cabin.
(there must be some otherwise the O2 levels would decrease and the CO2 levels would increase to intolerable levels fairly quickly wouldn't they?

20th Apr 2004, 03:57
There is no fresh air entering the aircraft without packs ON. It does not enter through any Fwd Outflow Valve, Flow Ctrl Valve or OEV.

The venitlation is from the differential pressure which is createdfrom the fully open outflow valve.

If you look at the B737 MEL for Unpresurized flight, it mentions ventilation by the OFV:

5. The following steps can be taken to improve both passenger-cabin and crew-compartment temperatures:
A. If possible, pre-condition the cabin to a low temperature prior to dispatch. A low initial cabin temperature will result
in a lower peak temperature.
B. Maintain the highest allowable flight altitude.
C. Minimize cabin heat by pulling the shades down and turning off unnecessary lights.
D. Position Gasper Fan/Recirculation Fan switches as follows:
-300/-500/-700/-800: Recirculation Fan(s) AUTO.
E. Because of reduced ventilation rate (approximately 1/6 of normal), consideration should be given to flight duration
and passenger load so that excessive cabin temperatures may be avoided.