5
$\begingroup$

In July 2008 a Qantas 747 suffered decompression at 29,000 ft and the pilots then descended to 10,000 ft in the space of six minutes . More details here : wikipedia.org/.../Qantas_Flight, 30 .

I'm curious to know if you're in a large aircraft descending at that rate would there be a strong sensation of descent for the passengers? Would the plane be angled downwards in a way that wouldn't normally happen or some other sensation not normally experienced when travelling on large civil aircraft?

I appreciate that in this case the masks were down and there might have been noise/wind associated with the actual incident, so clearly it did feel strange to these passengers, but if the descent was done as a test at that rate would it feel strange?

$\endgroup$
  • 3
    $\begingroup$ Yes, it would be noticeable. 19,000ft in 6 minutes is roughly 3166 feet per minute, or ~53 feet per second. Noticeable, although not overly aggressive if that is what you are asking. $\endgroup$ – Ron Beyer Feb 8 '16 at 21:23
  • 2
    $\begingroup$ @RonBeyer 3000 fpm is a standard descent rate in a transport category jet. $\endgroup$ – casey Feb 9 '16 at 2:57
  • 2
    $\begingroup$ @casey Right, it's a noticeable decent rate but not aggressive, I didn't suggest it was outside normal procedures. $\endgroup$ – Ron Beyer Feb 9 '16 at 3:35
16
$\begingroup$

From the physical side, you can not measure / feel if you are at a constant altitude or if you are ascending / descending with constant speed. What you can feel is the change of speed in any direction, and acceleration acting in an unexpected direction. This causes the usual sensations:

  • Accelerating / braking on the runway: You are pressed into the seat / belt.
  • Weird feeling right when the aircraft becomes airborne: The aircraft accelerates upwards, which presses you into the seat with more than 1g. This higher g-force feels strange.
  • While climbing, the aircraft still accelerates, but the more strange feeling comes from the fact that the force doesn't point down to the floor of the aircraft, but more backwards. Your eyes see that you are sitting upright in the aircraft, but it feels more like lying.
  • At some point, usually when the flaps are retracted, the climb rate decreases suddenly and it feels as if the aircraft falls. This is because the aircraft accelerated downwards - tough it still climbs, but at lower rate.
  • Usually, you don't feel much during a coordinated turn. Maybe you feel a little heavier, but when you look out of the window and don't see horizon, this again gives a strange feeling.

Of course, the passengers of that flight did have several sensations. right after the incident, the aircraft for sure has done some unusual movements, plus what the pilots did.

The reference #7 of the Wikipedia article contains this diagram on page 36:

Chart of flight data

Directly after the incident, the aircraft climbed a little and the speed brakes were deployed (50%) to reduce speed. This was for sure noticeable by the passengers. And I guess the aircraft started to descend faster than usual, so the sensation of "becoming lighter" will have been much stronger/longer than usual. After this, the aircraft descended with a quite constant rate for several minutes. This doesn't cause the feeling of being lighter. Maybe the attitude was unusual which means gravity doesn't push you vertically into the seat.

Later, when the aircraft reached 10,000ft, it accelerated upwards to stop the descent. At this moment, the passengers felt heavier.

(If you have a closer look, the descending part isn't a perfect straight line. This wasn't a smooth ride, but as said, a constant descent rate doesn't cause a sensation)

Of course, this is just about the sensation of acceleration so far. The passengers for sure have noticed the drop in cabin pressure, drop in temperature, more noise and so on.

$\endgroup$
  • $\begingroup$ I would imagine it felt a bit bumpy though due to the speed brake, like driving on gravel. $\endgroup$ – biziclop Feb 9 '16 at 12:52
-1
$\begingroup$

Yes, it would feel like an elevator descending very quickly. Elevators in small buildings generally descend at around 500ft/min. As an aviator you're taught that this is a comfortable descent rate for almost everyone, and it's what most General Aviation (GA) pilots shoot for. Elevators in most skyscrapers operate up to 2000ft/min. At 2000ft/min a human body reaches roughly one-fifth of its free fall velocity, making a person feel 20% lighter. This is a noticeable and sometimes nauseating effect. The other way that people feel descent rates is through pressure changes, often making ears pop and hurting young children. This isn't as much of a factor for commercial flights as their pressure changes only slightly above 10,000ft MSL.

I mentioned that most GA aircraft ascend/descend around 500/ft min for comfort. Most turbine or commercial aircraft will ascend/descend on instrument clearances that call for rates of 1000ft/min - 1500ft/min.

The pilots in the Qantas case did a great job. They achieved the perfect glide slope for that aircraft and conditions. Gliding that plane for 20 minutes would have resulted in a splashdown for most pilots.

Anectdotally, I am a pilot and have descended at 3000+ ft/min and can tell you it is VERY disconcerting if you're not conditioned for it.

$\endgroup$
  • 13
    $\begingroup$ Actually, you don't feel lighter when ascending/descending with constant rate. in a lift, you feel heavier when it accelerates upwards, then feel your normal weight while moving with constant speed, and then lighter, when the lift stops. You may notice pitch and of course change of air pressure, but from the movement alone, you can only notice change of speed in any direction. $\endgroup$ – sweber Feb 9 '16 at 0:14
  • 1
    $\begingroup$ Welcome! "This isn't as much of a factor for commercial flights as their pressure changes only slightly above 10,000ft MSL" No sure this is correct: What is the pressure in a civil aircraft fuselage at flight ceiling?. $\endgroup$ – mins Feb 9 '16 at 0:37
  • 1
    $\begingroup$ I don't agree with parts of this answer. In an aircraft, descent rate is relative, descent angle is what is important. A few minutes on Flightradar24 confirms that at the top of descent (and importantly, at near-cruise speed) a rate of -3000ft/min is common in airliners, giving a descent angle of circa 3 degrees. But as the speed slows for landing, the descent rate also lessens in order to maintain the same 3 degree angle. The passengers don't really notice the change in descent rate, they just notice the angle. I certainly wouldn't want to do -3000ft/min in a Cessna at 80 knots! $\endgroup$ – Ben Feb 9 '16 at 7:43
  • 6
    $\begingroup$ This answer seems to be suffering from a confusion of speed and acceleration. 3000ft/min descents are reasonably common for commercial airliners and pose no problems whatsoever - after the initial 'push over' there's no feeling of 20% lighter at all. $\endgroup$ – os1 Feb 9 '16 at 8:27
  • 1
    $\begingroup$ @mins That's incorrect. The presence of wind is not going to affect ones' perception of a descent. Also, the perception doesn't depend on the speed once the initial push-over is done - as I said. $\endgroup$ – os1 Jun 15 '16 at 6:31

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.