Yeah, that’s the sound.
Actually, a BAE146 (AKA a “Whisper Jet” or more lately a “Regional Jet”) makes a sound that is very similar and is very noticeable to the passengers, but it is an airflow noise produced during flap retraction rather than anything to do with crashing. It is loud enough and unsettling enough that the flight attendants normally explain the noise during the preflight brief so the passengers don’t start screaming.
I’ve been on a flight from Seattle to Newark where that’s exactly what happened. O[sub]2[/sub] masks dropped about 30 minutes into the flight, and they did a powered descent to 10,000 foot level. Of course, they were so busy on the flight deck that they didn’t have time to explain what was up until we had leveled off.
Between looking around to see if the other passengers were freaking out, wondering if Stephen King was right about a leak in the hull causing a jet to pop like a balloon, and thinking that I’d had plenty of warning from all the bad airline luck I’d had before, I was also thinking “My God, they were almost right about that noise a crashing plane makes.” I was sitting close to the engines, and they were indeed very loud and increasing in pitch.
I’m afraid I don’t remember what kind of plane this was - probably a 737 or similar.
No doubt this was due the the need for a rapid descent. I have a friend who flies 747s who says that a descent rate of 10,000 feet per minute (approaching 120mph) can safely be done - but it won’t feel or sound quite like level flight.
If the flight was Seattle to Newark, then it’s likely that the plane was somewhat larger than a 737 (not that it matters much).
Okay, we need pilot141 or 1920s Style “Death Ray” in here on this. Maybe, as you say, but that is some really high sink rate or decent rate in an airliner…
Yes, you can get very high sink rates in an airliner.
To make things easy, let’s not “mix measurements”: airspeed always refers to the airplanes forward velocity and Feet Per Minute (FPM) refers to its vertical velocity.
In certain cases (rapid decompression, cargo fire) it may be necessary to get the airplane to a lower altitude as fast as possible. The procedure normally involves disconnecting the autopilot (if engaged), bringing the power to idle, extending any drag devices (speedbrakes or spoilers or whatever the airplane has) and initiating a moderate bank (this is done to get the nose down without “pushing over”). Once the nose is pointed down you use pitch to control your airspeed: accelerate to Vmo/Mmo and then maintain it.
What this means is that you can get some VERY high descent rates initially. Let’s say you’re cruising along at 30,000 feet and .75 Mach. You get a rapid decompression and start the procedure for getting the airplane down. This particular airplane has an Mmo of .82 Mach, so you dump the nose and pick up airspeed until you’re indicating close to .82 Mach. With the nose down and the airplane accelerating (with drag devices extended) you can easily exceed 10,000 FPM on the VSI. I’ve personally seen 16,000 FPM (in real life in a C-141) and just over 12,000 FPM (in a simulator flying an MD-80).
Once you reach your max airspeed the descent rate will stablize at something less extreme - maybe somewhere in the 8,000 FPM range. At least one airplane that I’ve flown has had this descent rate limit it’s max certified altitude. The wing and engines were capable of going higher but in the case of a rapid decompression the airplane could not descend fast enough to a habitable altitude, so a lower “highest” altitude limit was placed on it.
And yes, this would feel different to the passengers - the intial bank and lowering of the nose accompanied by the speedbrakes would let you know that something abnormal was going on. (Assuming you were just doing an emergency descent, and did NOT have a rapid depressurization - if that happens there is probably a hole in the airplane and all bets about noise are off.) Once established in the descent the only thing noticeable might be a rumble from the speedbrakes and a lower pitch attitude - nothing really dramatic. The engines would be at idle so there’s no “screaming/plummeting to the Earth” movie noise and the wind noise is concentrated at the front of the airplane, so in the back it would sound normal to maybe just a touch louder. You would notice the retraction of the speedbrakes and the leveloff at your new altitude might be more abrupt than you’re used to, but all in all it’s not a gut-wrenching experience.
What Engywook most likely heard on his/her rapid descent was increased airflow noise over the wing and engine nacelle area. Remember - the goal is to get the airplane to a lower altitude as quickly as possible. This invloves trading potential energy (altitude) for kinetic energy (speed). You want to make the tradeoff as efficient as possible, because your airplane has a speed limit. Increasing power on the engines adds to your kinetic energy with no potential energy tradeoff (other than the amount of fuel burned, which slightly decreases your weight). You would reach your kinetic (speed) limit quicker without getting rid of as much potential energy, increasing the time taken to descend.
Hope this clears some things up!
Just as a matter of interest, our rapid descent procedure is a bit different in the Dash 8.
We leave the autopilot engaged, set and arm the “habitable” altitude we want to go down to–normally 10,000’–and set the pitch mode to IAS. Then we reduce the power to flight idle and increase the prop rpms to max (1200rpm.)
For the unitiated, this means that 10,000’ is “armed” so the autopilot will automatically level out at that altitude. By setting IAS (Indicated Air Speed) as the autopilot pitch mode, it will automatically climb or descend the aircraft to maintain the speed we have set. Because we reduce the power to idle and increase the props to max rpms, a lot of drag is created so the aircraft wants to slow down, at the same time, the autopilot noses the aircraft down to maintain the IAS thereby initiating the descent.
Provided we are satisfied that the aircraft is still structurally sound, we then increase the desired IAS up to the maximum permitted. The non-flying pilot (normally the First Officer as our company ops require the Captain to take control for an emergency descent) then calls out the altitude at 20,000’, 15,000’, 12,000’, and 11,000’. Checklists and trouble shooting are then carried out at the safe altitude.
I’ve only done a couple of emergency descents in the simulator and I must admit that I wasn’t looking at the vertical speed indicator at the time so I don’t know what descent rate we got to. The max cruise altitude in the Dash 8 is 25,000’ so it all happens pretty quickly.
There was a B767 conducting an ILS into Adelaide a while ago. The ILS wasn’t serviceable though (the crew had neglected to check the NOTAMs and ATC had cleared them for an ILS, it was a clear day.) The ILS was radiating a test signal which was giving the aircraft autopilot a false “fly down” command. The aircraft descent rate got up to 6,000 fpm before the situation was rectified by the crew. And that was just tooling around at approach speeds.
P.S. We don’t carry passengers so “pushing over” to get the nose down is not a problem.
Wouldn’t it go to high (coarse) pitch?
Not a nitpick, I’m genuinely asking.
I’d heard that some airplanes used the autopilot on a rapid descent - neat to find a specific example!
Way cool info guys… I had no idea.
I have been on fire up high and in my little airplanes, well, you can’t get down that fast in one piece.
I also was under the impression that high sink rate with full flaps in a “T” tail while in a bank and even more so with a bit of cross controlling was not a good thing.
I have had some small planes object to that treatment.
Once the propeller pitch lever in put into the fine pitch position, it won’t go to a larger pitch unless there is output from the engine driving it. ( It is out of it’s operating range )
The engine can be over revved ( piston types, don’t know about new turbine applications ) if a steep / fast enough decent / airspeed can be achieved. ( they act just like a fixed pitch prop in those cases. ( has to do with the physical way they are built. )
With the power at idle the prop should go to fine because the rpms will be low and it will be trying to increase them. If you are in a dive, the windmilling prop will increase rpms as speed increases (the same as a fixed pitch prop would.) Once it got into the governing range of rpms, from about 2000 up, it should start to go coarse again to maintain whatever rpm you had initially set for it.
This is different from an engine failure situation where you have lost oil pressure. If you lose all power to the engine and have no oil pressure the prop will either go to coarse or fine depending on the design.
Stalling and spinning can be a bad thing in a T-tail because the disrupted airflow from the wing can blanket the tail plane taking away your elevator authority, and as you know, it is pretty vital to have some elevator control to recover from a stall or a spin. So, full flaps, turning, and out of balance at a low airspeed is not a good thing in that you are close to a wing-drop stall, but then that is a bad thing in any aeroplane.
With modern airliners having stick-shakers, stick-pushers, and even angle of attack limiters (Airbus) it is pretty difficult to inadvertantly stall.
With an emergency descent, your airspeed is high so there is nothing particularly dangerous about it providing you level out somewhere above ground level :).