747-400 Freighter Fire Suppression

[NSEU]

According to the operations manual, if the Main Deck Cargo Fire ARM & DEPRESS switches are pushed, the aircraft goes into a partial decompression.

Two packs are shut down and the Main Deck cabin altitude target becomes 25,000 feet (controlled by the CPC in command). However, the manual says that the Upper Deck enviroment is comfortable.

How is this possible? With, say an Upper Deck cabin altitude of 8,000'(?) and a Main deck cabin altitude of 25,000', I estimate a differential pressure of 5~6psi. The force on the upper deck door at the top of the stepladder would be over 10,000' lbs.

Have I miscalculated? (or overlooked something)

Thanks.
Cheers.
NSEU.

[john_tullamarine]

Supposition ...

I suspect that the aircraft goes to 25000 feet, top and bottom. This is the generally accepted level for physiological acceptability .. although probably more OK for younger folk than the older chaps ... and provides for reduced combustion in most cases .. may even snuff out the fire.

The crew would then be on masks for the duration.

[Intruder]

There is no way to decompress only the main deck. JT's supposition is correct.

OTOH, if you haven't worked in an actual cockpit pressure of 25,000', even on 100% O2, you'll find it's less "comfortable" than "tolerable."

[NSEU]

On 747-400's there is a common plenum for all 3 packs, however, a baffle in the plenum ducting tends to direct more of Pack 1 air to the flight deck (if the other packs fail, plenum air can go around this baffle to the other zones).

I was originally thinking that this baffle system would provide the extra "comfort". However, on the Freighter, there is some kind of flow regulating valve between the plenum and the flight deck. Looking at the wiring schematics, the valve appears to respond to the Main Deck Cargo Fire ARM switch (it seems to run the valve to the fully closed position .., but it's difficult to tell from the schematics. It may do exactly the opposite).

I'll see if the Maintenance Manual has any more information on this.

Anyway, thanks for the feedback.

Cheers.
NSEU

[Intruder]

While the direct output from the pack may go to the flight deck first, there are no pressure bulkheads between the flight deck and the outflow valves. In fact, numerous blowout doors will open if there is any significant pressure differential between compartments. Once the outflow valves open and regulate the cabin pressure to 25000', that is the pressure altitude for the entire airplane. The only "comfort" part of it is that the temperature of the flight deck can be regulated directly, but the main deck will likely stabilize at a much lower temp.

[BelArgUSA]

I suggest you do not "generalize" among 747 freighters.
Used to fly 747-200/300s... There were differences with 747 freighters.
Those known as "747-xxxF" which were "real freighters".
And those converted from passenger aircraft "747-xxxSF" special freighters.
Often these "SF" were originally equipped with the SCD Side Cargo Door.
xxx
Same thing happened to to the 400s...
There are differences with air conditioning and fire protection.
Some "conversions" are not entirely similar to "original" freighters.
All depends who made the conversion, and what the customer requested.
xxx
Have a look at number of packs, lower lobe fire protection, and other items.
As usual, if you study any 747, be specific as to "which one" you study.
Besides that, there might be weight limitations differences too.
Differences are just not the nose cargo door.
xxx

Happy contrails

[Intruder]

The parts about lack of differential pressurization between cockpit and main deck are common to all 747s (except the "guppies" flown by Evergreen, where the main cargo compt is unpressuruized) -- it can't be done.

Other than that, the question was specifically about the -400, so there's no conflict with any Classic info.

There are no relevant differences in Air Conditioning and Fire Protection between the straight 747-400F and the 2 different conversions that we fly.

[BelArgUSA]

Note to NSEU -

Hola NSEU -
xxx
A severe pressure differencial inbalance between Upper Deck and Main Deck...
Would lead to a complete (structural) collapse of the floor.
xxx
I have flown 747F, 747C and 747SF. Does not matter if not 400s.
A collapse of the upper deck floor would would impede/destroy cables.
Cables linking flight deck to all tail flight controls hydraulic units.
xxx
Relief panels are installed between the three decks.
Upper, main deck and lower lobe.
What "book" are you reading...? Only read Boeing or airline own manuals.
When I puffed my Marlboros in the flight deck, the smoke went to outflow valves...
Under the plane's belly, through each separate floor...
xxx

Happy contrails

[NSEU]

Thanks again, gentlemen.

I did have great doubts that any significant pressure differential could exist, but the way the manuals are worded leads to confusion. As previously said, 25,000' is not what you would call comfortable.

After further reading, I discovered that the Flight Deck (aircon) Flow regulating valve is fully opened, rather than fully closed during a cargo fire, so perhaps there is a very slight positive differential which (helps) stop smoke entering the flight deck.

Cheers.
NSEU

[SMOC]

Is this a Boeing FCOM you are reading or a reproduced company manual?

[NSEU]

Is this a Boeing FCOM you are reading or a reproduced company manual?

The guy who gave me the "comfort" quote had a US-based freighter airline's power point (pilot) presentation. I used the same airline's maintenance manuals and wiring schematics to dig a little deeper into what was happening.

Unfortunately, there were still big gaps in our collective knowledge

Cheers.
NSEU

[SMOC]

I think the use of "comfortable" is an assumption based on the fact conditioned air is still available to keep the cockpit temperature reasonable. As others have said it won't be overly comfortable but it's the effect on the fire that's the goal.

[LLuke]

I think it mainly reduces Nitrogen bends...so you don't become incapacitated.

[john_tullamarine]

reduces Nitrogen bends

25,000 ft shouldn't cause decompression sickness.

However,

(a) as the divers amongst us well know, flying (especially in the event of a decompression) following diving ... is an absolute no-no, just for this reason. Recollection suggests that a day clear in between is the general rule ?

(b) folks with lots of dental fillings may have problems with toothache

(c) folks with colds or just sinus problems may have significant problems. I recall a 25,000 ft (or thereabouts) chamber run years ago in which we had several divers along for the exposure experience. One had been diving a bit too soon before and, while he didn't end up with any joint problems, he did have a blocked eustachian and he was not a happy little camper at all ... the tears associated with the pain just about flooded the chamber. Fortunately, the flight surgeon was able to clear his ears on the way down and all was well.

[Intruder]

Remember that 25,000' chamber runs (and their 40,000' predecessors) are normally done with young, healthy military pilots, not older, sedentary airline pilots. Response to high altitude is VERY variable with physical conditioning, or lack thereof.

While I once thrived at 8,000', I can feel the effects of 6,000' these days. Even 100% O2 at 23,000' was a bit of a chore 15 years ago...

[LLuke]

Next to health and pressure altitude, also time is a factor. Couldn't find any examples unfortunately...

[BelArgUSA]

As usual - Advanced Aeronautical Sciences at Wikipedia Institute helped.

Time of Useful Consciousness - Wikipedia, the free encyclopedia

What Wilipedia does not say, is that above 62,000 feet, you blood will boil.
I used to scare myself in F-104Gs... 1964-1968.
Some idiots dashed above 55/60,000' - without pressure suits...
xxx
Opinion of passenger oxygen masks above 25,000 feet = WORTHLESS...
Oxygen masks for pilots (747) - Might save you up to 33,000 ft.
But get lower ASAP...
xxx

Happy contrails

[point8six]

There will be no pressure differential between upper deck and main deck, following activation of the Main Deck Fire arm and depress switch (otherwise it would be impossible for crew member(s) to descend to the main deck to check/fight the cause of the warning). As to the "comfort", No.1 pack will probably struggle to maintain a comfortable temperature in the cockpit and upper deck area (depending on the O.A.T at 25,000ft). The temperature on the main deck will probably decrease towards O.A.T. (unless fire suppression was unsuccessful!).
It is a "plan to land at nearest suitable airfield" checklist (ditching may be necessary), which is why there is a lack of time information.
- the above refers to the 747-400F.

[violator]

Pressure suits

Your blood won't boil above 62,000ft / above the Armstrong Line or in space. For a normal blood pressure of approx 120/80 (gauge), 80 Torr = 10.7 kPa. At 10.7 kPa water will boil at 42.3 deg C, which is above body temperature.

Though the above is generally true, there will probably be localised regions where the pressure falls low enough small pockets of vapour to form, causing the vapour pressure to rise. Water in soft tissue will boil which would be pretty painful, but you wouldn't explode.

The problem would be getting to this point without blowing out your lungs or intestinal tract, and staying conscious. Don't try at home.

Human Body in a Vacuum

[muduckace]

BelArgUSA

Hola NSEU -
xxx
A severe pressure differencial inbalance between Upper Deck and Main Deck...
Would lead to a complete (structural) collapse of the floor.
xxx
I have flown 747F, 747C and 747SF. Does not matter if not 400s.
A collapse of the upper deck floor would would impede/destroy cables.
Cables linking flight deck to all tail flight controls hydraulic units.

While I agree that generally pressure would remain the same through out the aircraft, I disagree that the floor would collapse. The aft "cosmetic" bulkhead of the upper deck is the weak point. It would also suck that wimpy upper deck smoke door right out.

Been years since I have been on a 747, I remember some classics with a smoke valve that I thought connected to a tube to connect to the main deck to dump overboard, I don't know if boeing thought it would reduce the amount of smoke below the cockpit.

Those packs put out more air than any other aircraft I have been on, I remember closing the out flow valve with the doors closed on the grund and the overhead hatch open and burning a cigarette at the F/E panel, I could flick my ashes and they would go straight up and out.