B737NG unpressurized
 

Hi,
The MEL for unpressurized flight contains a chart that gives the cabin temperature and the flight deck temperature after 30 minutes of unpressurized flight with 45 POB. ( the max for dispatch).
With a TAT of -1 Celcius cabin temp = 21 degrees and the flight deck 24 degrees.
With a TAT of 10 degrees the cabin temp is 27 and the flight deck 35 Celcius.
I would appreciate your best guess on three questions:
1/ What is the TAT at 220kts at 10,000ft on an ISA day?
2/ What would the above figures increase to with 170 POB?
3/ What would the above figures increase to with 170 POB and 60 minutes duration instead of 30?

Hello again,
No response after six weeks surprises me, I would have thought the 737 guru's in Pprune land would have found this interesting.
I am still unsure of how hot it would get in the case of a dual pack failure. The QRH makes a recommendation to maintain a minimum of 290kts once down at 10,000ft . This recommendation seems to be at odds with the MEL assertation that TAT is an important factor in cabin temps in this scenario.
Come on you knowledgable folk.......what's your opinion?

Part 1 of the Question...
 

According to an old rule of thumb, 220 KIAS at 10,000 ft, ISA, should yield a TAT of about +2 deg. C.

I have done an unpressurised flight in a Classic. It does get very warm very quickly though my flight was empty to position to a maintenance base, I would not like to try it with pax.

A friend of mine did a 2.5 hour flight to recover from a Pacific Island in a classic many years ago. They were down to undies for the last hour due to the heat.

What is the primary cause of heat buildup, the electrical installation in the panel or the greenhouse effect from cockpit windows?

I imagined there was an air purge vent designed to clear smoke at low altitudes that could cool the flight deck?

There is no air purge. If you have a confirmed source of fire on the flight deck and are carrying out the smoke removal checklist there is instruction to open the first officers window.....that's about the closest to an air purge I can think of.
As far as the primary source of heat goes, I imagine it is a combo of both. One checklist tells you to pull window shades and turn Galley power off so that is covering both elements you mention. I would have thought the pax themselves would be a major source of heat. The MEL for unpressurized flight limits the number of pax to 45 presumably for that reason?

How bout Recirc fans OFF for smoke removal?

Many years ago as an aero eng student I visited the Concorde thermal test lab at Farnborough during which we were told that humans radiate about 110-120 watts/hour. Hence the need for cooling.

They obviously never met my wife :p

Regarding the original post, it's all a bit pointless, isn't it? I can't imagine any reputable airline operating unpressurised with pax except in the most dire emergency (Hollywood film stuff like zombie apocalypse).

I see what you mean shuffler, but I can imagine most reputable airlines expecting the Captain to operate the aircraft on the MEL with one pack inoperative. In fact, it is something that most Captains will probably see in their career. So, with that being the case, it would be nice to know what the cabin temp is going to be on the off chance that your one good pack lets go.
If I turn up to work tomorrow and am flight planned at FL 250 with a pack out, when we are over an hour from the nearest airport and 170 POB I would like to have an accurate idea of the consequences of the one remaining pack malfunctioning.
I agree it is not highly likely, but I don't think it's quite zombie apocalypse Hollywood stuff.

Hi Framer, totally agree it would be good to have an idea of the likely temperature. The following is a simplified formula which may help:

TAT = SAT (1 + [0.2 x M2])
where,
TAT and SAT are expressed in degrees Kelvin (Kelvin = Celsius + 273.15)
M2 = Mach no squared (sorry, couldn't work out superscript on iPad)
0.2 is a simplified factor which includes ratio of specific heats and assumes an empirical recovery factor for the probe of 1.0.

So, 220 KIAS at 10,000 ft in an ISA atmosphere:
SAT = -4.8 degrees Celsius = 268.4 degrees Kelvin
M = 0.4
Using the formula above,
TAT = 277 degrees Kelvin = 4 degrees Celsius

From the MEL, I guess that means the flight deck could be at around 29 degrees Celsius. From what I hear anecdotally, the flight deck does get quite uncomfortable so that could be correct. Hopefully the maths is correct........

Thanks a lot SHSS, great info.
Still unsure as to the difference an extra 130 pax and an extra 30 mins would make. Quite a lot I imagine.
Cheers

Hi framer, very good question

You will have no problems to control temperature even in tropical weather.

The more pax the better.

The secret may be in Ram Air Valve that may not be mentioned in a check list.
Whenever you feel warm, open that valve and you will be a witness of the most efficient , most silent, most lovely fresh air you can imagine.
Just do not keep it open too loooong.

I think you may have opened your secret ram air valve a bit too often GG :eek:

A fair point, and I have had to operate single pack a few times. Practically speaking, though, if you lose the other pack, it means that you are going to conduct an emergency descent and then fly at FL100/MORA while diverting to a suitable field (you could continue to destination if it isn't far, but fuel consumption and the possibility of barotrauma injuries and pax panic are going to force a diversion in most cases). So the flight time is likely to be relatively short and the FL fixed. The only thing you can really do if needed is slow down to open a window with the outflow valve open to get enough through flow. I think trying to pre-calculate temperatures is over-analysing things.

A rapid descent for sure:E

Yes I agree. You would be unlucky to be an hour away from landing as you won't be EDTO if you dispatched that way, however, there is a big difference between 30 minutes with 45 pax onboard and 60 minutes with 170 POB.
Fair enough.every brain is different. I'm not actually trying to have an exact figure in my mind, I'm fishing for information as to weather it will be intolerable from someone who knows. Maybe nobody on this site knows and all we'll get is guesses, that's fine too, I enjoy the process :)
My best guess is that it will become intolerable and we would in fact end up slowing and opening the F/O's window.

I think you're right - I'd guess that after 30 minutes, it's going to get pretty uncomfortable and after 45 severely so. I don't know how much difference pax load will make - obviously some, but I suspect most of the heat is generated by aircraft systems. Shutting the blinds on the sunny side and switching off as much equipment, especially in the cabin, as possible will help.