B747-8 Cargo Fire

[whale2guppy]

can anyone explain to me why on the 748 does the QRH descent at MMO/VMO? On main deck Cargo A/C both the 400 and the -8 call for high speed descent? Can anyone help me understand the need for high speed (other than the obvious need to land ASAP)

[RandomPerson8008]

The need to land ASAP is the reason. You may only have about 15 minutes to live unless you get it on the ground if the fire is real. If you are very close to the airport you want to land at when the fire warning occurs, you'd have to use your airmanship to determine the fastest way to get the airplane on the ground by manipulating the speed and configuration of the machine. If you're directly above the airport you wish to land at, it might be prudent to spiral down with gear extended and speedbrakes up. I don't know how much I would respect the gear extension limitations in a fire situation, especially if smoke/fumes were becoming visible, but it is preferable not to damage the airplane anymore than necessary. I think you will get a higher rate of descent with gear down at 320/.82 than gear up at Vmo/Mmo (365/.90). Study UPS6, UPS1307, Swissair 111, Asiana 991, Saudia 163.

[back to Boeing]

whale2guppy

it’s purely that. Every second counts when you’re on fire. You descend down to the mid 20’s as a trade off for max TAS vs minimum oxygen to feed the fire. Then when you’re 75 miles away from your intended runway you descend at VMO and do an idle descent flinging out the services from 3000’ onwards at the placard speed and you “should” still be stable at 1000’ (although that’s a secondary consideration but screwing up the landing and going off the runway when you’ve made it that far). More info is available in the FCTM.

[Avinthenews]

RandomPerson8008

I would not go lowering the gear too far above the limit speed. You rip those doors off you may cause a hydraulic failure and who knows what other damage.

[deltahotel]

+1 to all that.

Stay at 25000’ til it’s time to plummet. We did this as a sim scenario in 757 and 767 recently. The bravest/most optimistic just got in from 47 miles because forgot to press FLCH to initiate the descent at 55 miles.

[B2N2]

Consider Vx and Vy, one is over distance (angle) and one is over time (rate of climb).
Now take that same concept and consider a descent.
With a fire your main concern is time, the average hull loss as a result of fire has occurred shockingly fast.
Even if Mmo / Vmo may give you a shallower descent it’s at a higher speed therefore allows you to cover more distance then a descent with gear and flap hanging out at a much lower speed.
Its much better to have an aircraft safely on the ground that may have exceeded speed limitations then a hull loss in the air.
According to Boeing overspending is a much lesser issue then over stressing the airframe.

[Ozgrade3]

During class discussion about fire in flight. It was asked if a fire in flight occurred, would depressurising the aircraft above FL300 starve most fires (including electrical fires where the insulation is burning) of oxygen to put the fire out.

[MENELAUS]

That essentially is what the big red main deck fire button does. Depressurise the aircraft. Probably of little efficacy against electrical fires or arcing etc. Anything involving lithium is of course seriously bad news. So they’re now attempting containment with special barriers / ULD’s etc.
Anything involving lithium ion / lithium metal on fire and you’re buying the farm unless you can get it on the ground / in to the ocean, ASAP. Literally minutes.
The Dash 8 can carry in excess of a 100 tonnes of the stuff. Some airlines, not all, ban the carriage of Li Metal batteries, a factor I believe in the UPS 6 tragedy. They are less restrictive when it comes to carrying lithium batteries in devices; tablets, the latest phones, etc etc. that we all have to have instantly.

[negativeclimb]

But why the controlled depressurization brings the pressure environment to FL250 instead a fully depressurized aircraft? Better the last option, isn't it?

[MENELAUS]

Fully depressurising an aircraft whilst staying level for even a short period brings other problems. Not least the inability to breathe easily, ( look up pressure breathing, time of consciousness etc), see, function and the fact that you’re probably in your shirt sleeves at -58.
A dive to flight level 250 is a decent compromise whilst maintaining maximum forward Mach no. It’s also a good flight level when you get there for maximising true air speed as you turn towards your alternate; probably Shemya on a dark cruddy night lying some 1200 miles distant. And it’s manageable from the point of view of discomfort and o2 depletion.
The checklist makes the best out of a bad scenario; I can’t think of any case where this has happened where the poor unfortunates were able to survive. Happy to be proven wrong.
One of the many joys of hauling rubber dog 💩 around ( in fact that would be infinitely safer) whilst carriers try to maximize yield by transporting ever increasing amounts of this stuff.

https://www.aerotime.aero/27655-Hong-Kong-Air-Cargo-bans-VIVO-smartphones-after-airport-fire?v=amp

And that was a relatively small load.

[RandomPerson8008]

One time I did have to press the main deck cargo fire arm and depress/discharge switch due to a FIRE MAIN DECK AFT message. Unfortunately this was at about 30 west on the NATs. Not a good feeling. The depressurization is fairly controlled and the remaining operative pack did a fine job of maintaining cockpit temperature both during the descent form the 30's and at FL250. By the time we arrived at the nearest airport it was apparent that the fire warning was a smoke detector system malfunction, so the approach was relatively normal. An ACM on portable O2 was dispatched to the main deck to investigate and fight the fire if necessary. The grim prospect of ditching was of course going through our minds had the fire been real. A few "monday morning quarterbacks" claimed we should've have disregarded the warning due to the slim likelihood of a cargo fire igniting 4+ hours after takeoff. I think you have to take the warning seriously and o everything you can to get on the ground ASAP. Had we ignored the warning and it turned out to be real, we would've been posthumously ridiculed as muppets.

[Check Airman]

I hope I’m never in the back with those quarterbacks up front.

[Switchbait]

Amen......

[MENELAUS]

Unlikely you’ll be in the back of a freighter at any stage. Unless it’s as HUM.

[FlightDetent]

RandomPerson8008

Respect. Man's gotta do ...

Watched UPS6 from ACI just recently. Yet again a very unpleasant experience, dramatized or not.

[Nick 1]

RandomPerson8008

Those quarterbacks did not learn any lesson from Swissair 111.

[whale2guppy]

All responses much appreciated Thank you!!!

[cloudchsr]

There are probably multiple reasons for descending at VMO-MMO with a main deck cargo fire. The fact that you are on fire and want to land ASAP is a valid one. I think the core reason has to do with the fundamental philosophy behind how main deck fires are dealt with. Absent fire suppression agents, the fire-fighting method is based on oxygen starvation. The goal is to stay at an altitude where there is not enough oxygen to feed the fire until the last possible second, minimize time at lower (oxygen rich) altitudes, and get the airplane to a spot where a fire department can take over and fight the fire. The steepest available descent from that altitude is at VMO/MMO. Using that speed minimizes the time at lower altitudes which could feed the fire and allow it to get out of hand. Note that this will not help you if the type of fire is one which manufactures its own oxygen.