I had never heard of this incident, and wanted to comment on how absolutely miraculous it is that the wings stayed on this airplane. Transport category airplanes are typically certified to +2.5 G with the flaps up, and a quick google tells me that the 747 is the same. That airplane more than doubled its primary structural design limit without failure. I’m speechless.
Anyway, back on topic. As Whack-a-Mole correctly posted earlier, emergency descent procedure depends on the emergency. In the small corporate jet I fly, if there is no suspicion of structural failure, we bring the throttles to idle and roll the airplane a bit to allow the nose to fall through the horizon. The airspeed quickly builds, and we begin applying spoilers to stay right up at the limit without exceeding it. Our vertical speed indicator reads in feet per minute, and the gauge limits are +6000 to -6000. With the engines at idle, spoilers fully deployed, and pitched for maximum airspeed, the needle is buried at -6000. In the sim I can lose 30,000’ in somewhere around three minutes. I haven’t timed it, but that’s a reasonable estimate. We’d use this procedure for something like a cabin fire.
If we suspect structural damage (indicated by missing airplane pieces or a stiff breeze in the cabin), the key is limiting our airspeed and maneuvering loads so as to keep what’s left of the airplane in one piece. As mentioned up thread, the passengers have a ten minute supply of O2, so I have plenty of time to get down to a more habitable altitude.
Hopefully LSL Guy or ElvisL1ves or one of the other airline types will be by soon to enlighten us with their experience.
FWIW, the Canadian Aviation Regulations (and, I’m sure the USA’s Federal Aviation Regulations…the former is more or less verbatim the latter) has the following regulation in place; 525.841 Pressurized Cabins:
My understanding is that aircraft manufacturers are required to show that the airframe can handle a descent which will allow this criteria to be met, though I’m not sure how. If we consider a plane to drop about 30000 feet in 2 minutes, that’s a rate of about 250ft/min. I have no idea if other limitations affect that in real life, though, and I admit that I could be reading the regulation incorrectly!
I know that chapter of the CAR says transport category, so it probably doesn’t apply to the following, but what do they do for bizjets that regularly cruise at more than 40000 ft? Do they have proper pressurised masks for the passengers? I suspect regular airliner chemical masks won’t be adequate at those altitudes.
I sent the OP to a friend who is in a position to know about such things. This is his response:
The normal routine for an emergency descent is of course for cap and fo to get on O2 themselves first and establish communication over the interphone. The FO will try to control the cabin manually if quickly possible but will not spend alot of time trying. Once he clearly states to the cap “cabin uncontrollable” the cap announces “emergency descent”. All jets I have flown call for an immediate high speed descent with thrust idle and speed brakes deployed fully. You keep the speed at or near barber pole (Vmo or Mmo (max operating velocity or mach)) until levelling out at your target altitude (10,000 or lowest altitude for terrain clearance).
The point is to get down quick and usually you are descending in excess of 6000 '/min. The only reason to keep the speed back would be if you suspected structural damage. Delaying the descent to slow back to gear and flap operating speed is not recommended normally. The questioner referred to the operating time of 12 or so minutes for the oxygen generators - the reason for this ‘window’ is not so you can take more time to get down - that is done immediately and at a high descent rate - but to cater to the situation where you are over mountainous terrain and maybe you cant level off at 10,000 right away. You may have to level out at 15,000 or so for say eight minutes.
If you are flying a mountainous route you need escape charts that plot a track for you to get onto so you can get down to 10,000 safely. You should have these charts out and ready while you are in that zone. Incidentally your fuel load must cater to a loss of pressurization and subsequent diversion at 10,000 to a suitable alternate.
Transport Category airplanes are, in general, jets greater that 12,500 pounds or props greater than 19,000 pounds. Most small jets you see will be greater than 12,500 lbs, except for the really small ones. Even then, many of those are still designed and certified under the more stringent transport category regulations. The applicable FAA reg, 14 CFR §25.841, is pretty much exactly what mnemosyne posted, aside from spelling “airplane” differently. The wording allows service ceilings above 40,000 feet:
Most corporate jets have a service ceiling somewhere north of 41,000 feet, and regularly operate at those altitudes. Some newer airliners are also certified to or above 41,000’, but are rarely able to climb that high because they are loaded as heavily as possible with revenue-generating people and cargo. Revenue is not generally load-based for corporate aircraft, so they are usually lighter and can climb to the higher altitudes for better weather, traffic conditions, and sometimes better fuel economy.
Upon review, I see longPath posted an excellent overview. Specifically, I wanted to clarify the last point. The mention of planning enough fuel to make it to your destination at a lower altitude is important. True airspeed is altitude dependent, as is fuel consumption in turbine engines. The lower you are, the slower you go and the more fuel you burn. If I’m halfway between LA and Chicago and have a loss of pressurization that forces me down below 10,000 feet, my decreased range will force me to find an airport somewhere short of Chicago to land. Not a big deal. If I’m halfway between LA and Maui, however, this can be a real problem. I must plan enough fuel, considering the greatly decreased range, to be able to get to my destination or diversionary airport even if I’m forced down to 10,000 feet. If I can’t carry that much fuel, I have what is called a ‘wet footprint’ and I risk taking my passengers for a swim. The solution to the problem is to take a different route (with diversionary airports in range), or to just get a more capable airplane.
Haha. I’ve mentioned this before but I play a game when opening some threads. I guess at what post I’ll see a given answer (or I’ll post it myself if it’s not there yet). In this case I guessed that by the 2nd reply was when this would be answered with 32 feet per second per second (or 9.8 meters) .
Alas, I’m sad to say you let me down runner pat, as you were (I believe) the 5th responder.
You need to work harder on those types of answers because if were left up to me, I would have been what, the 20th reply? And that’s much too late for good answers like that.
Yes, I saw a DC8 that did this many years ago. Mind you, I as half-way across the city and the only thing I could make out was the big ball of flame in the sky after it dropped over a hundred feet, left an engine on the runway, and the crew tried to do a go-round. It travelled across the sky for about 30 seconds before it fell from sight in a big ball of smoke and flame. IIRC about 140 people died.
I read an accident report about it on the internet several years ago. Apparently the brakes handles worked in 2 different ways depending on models, and the copilot was familiar with version B, which meant he deployed thrust reversers way too early. His last words on the voice recorder were “I’m sorry, Frank!”
Which is the same regulation, which makes sense, given that the limit has nothing to do with the aircraft itself but rather with the lifespan of humans in an atmosphere with insufficient oxygen!
I think the general regulations regarding the two classes of aircraft are the same; it lays out a minimum to which an aircraft must be certified, but does not prevent a manufacturer from doing more. Business jets will have oxygen onboard for all passengers, with a limited supply because to carry more than *necessary *adds weight to the plane that isn’t desirable. Note that necessary will take into account the higher service ceiling, so it might actually amount to more on-board oxygen.
I’m sure that’s correct for most applications, but because weight is such a concern for aircraft the standards are actually a bit less stringent. The 14 CFR Part 25 certification regulations specify the positive load limit for transport category airplanes be at least 2.5G, and that the factor of safety be 1.5, or 150% of design load limit.
That means that the wing of a transport category airplane (any jet above 12,500 lbs or any turboprop above 19,000 lbs) must be able to support at least 3.75 times the aircraft’s weight without failure. The NTSB report on that incident states the airplane weighed 440,000 pounds at the time of the incident. So, we take a wing, bring it to its absolute designed maximum load limit, then add nearly 600,000 pounds. That’s impressive to me. Or maybe it’s the fact that when I teach loops, we typically target a 4G pull at the bottom. Imagine the possibilities!
Airline type here … **StrangerThanFiction **& **longPath **have covered current big jet practice pretty well. The [slow down & hang gear & flaps then descend method] is not current practice. You don’t get that much advantage in descent rate (if any), and the delay to slow happens up at the top where survival gets increasingly iffy in a matter of seconds.
We figure 7000 feet per minute (FPM) as a ballpark average descent rate for DC9/MD80 & B757/767 aircraft. We lose a little off the max rate getting started down & a little leveling off at the bottom. But 7000 FPM is a decent average.
Big picture, from “Shit, we gotta get down now” to “we *are *down now.” takes 3-1/2 to 4 minutes. Assuming structural integrity. If not, figure more like 6-8 minutes.
The actual deck angle is about 10 degrees down at most. It just feels real steep since you rarely see more than 4-ish degrees deck angle down in normal operations. It’s easy to get going too fast if you get the nose buried. Overspeed is as lethal a mistake as the decompression you’re trying to escape.
As to approach to Burbank, there’s nothing particularly unusual about Burbank procedures. On any particular appraoch to any particular runway ATC or other factors can drive a need to stay higher or faster than normal until later than normal in the approach.
Normally for fuel savings we extend the gear at the latest possible time which is also as low & slow as possible. Which in turn means least possible noise & vibration in the cabin. If circumstances leave the aircraft high and/or fast, the only way to catch up to the desired descent profile before touchdown is with gear and/or spoilers deployed at higher speeds. Which leads to a more nose-low and vibration-filled descent until the normal path is intercepted.
Disconcerting to the folks in the back, but not a big deal as evidenced by the unconcerned pilot passenger. There are still vast margins before anything dangerous starts to happen.
The early DC8s could inflight reverse the inboard engines to idle power settings. Those aircraft didn’t have effective speedbrake/spoilers. The design of the reversers on those engines was entirely internal to the engine, so there were no exposed deflectors sticking out into the free stream.
Unlike the exposed deflectors you see here File:Reverse.thrust.klm.fokker70.arp.jpg - Wikipedia . The exposed deflector types generally are not reversable in flight because once the deflectors are out in the free stream airflow there isn’t enough power in the mechanism to stow them agaisnt the force of the wind. In other words, they’re stuck in reverse until you stop moving one way or the other.
The DC8 accident alluded to above was a case of some airplanes in the fleet having one kind of reverser and some had the other. If the air/ground interlocks weren’t configured correctly (or hadn’t been invented yet), it’d be possible for the pilot to deploy the non-retractable type. And inevitably, somebody somewhere made that mistake. And died for it.
