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A few days ago I was on a flight from SFO to AUS (Airbus 320) and I was shocked by the steep angle of descent into AUS. I have been on over a hundred commercial flights but I've never experienced such a steep descent. I checked the flight on flightaware.com and noticed that the descent rate was 3,000 to 4,000 fpm. On other days the descent rate is closer to 2,500 fpm.

Flightaware data: My flight vs another flight.

Two questions about this:

  1. What is considered a "normal" descent rate for the A320?
  2. What could be the reason for the exceptionally steep descent compared to a normal flight?
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also consider it against horizontal speed, 4000 fpm is 45mph (70 km/h) but the horizontal speed is 10 times that –  ratchet freak Aug 22 at 18:32
    
Sure, but the angle of descent was exceptionally steep. GS was 480 kts with a descent rate of 3,900 fpm. On other flights it's 460 kts and 2,500 fpm. –  Philippe Leybaert Aug 22 at 18:41
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The old slam-dunk approach perhaps? –  falstro Aug 22 at 18:56
    
The flight was 20 minutes behind schedule, so that may have saved them 5 minutes or so. –  Philippe Leybaert Aug 22 at 21:10
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@Mehrdad The plane was in a steeper than usual nose-down attitude, engines idle and spoilers deployed to keep the speed in check –  Philippe Leybaert Aug 23 at 15:32

5 Answers 5

up vote 8 down vote accepted

There are a few possibilities, there's no way to know which is correct without more information:

  • There could have been an emergency on board. If someone is showing signs of a serious illness pilots will get the plane on the ground in a hurry to get the person medical attention as soon as they can. Or there could have been an issue with the airplane or a passenger, like someone very drunk and unruly in first class, say
  • As @falstro said it could have been a slam-dunk approach, where a controller directs airplanes to do steeper descents to solve sequencing issues. Pilots know if they say "unable" that they probably won't get another approach for awhile, so they'll accept a steep descent to get on the ground sooner
  • It could have been a request of the pilots for a non-emergency reason, like them trying to make up scheduling time, or to stay within fuel limits
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7  
I'd add to the above a desire to keep as high as possible as late as possible due to possible small arms fire. We did this in the 1990s into Bosnia and Croatia. –  Terry Aug 22 at 19:19
    
It makes sense for scheduling time, but not fuel limit. With glide ratio around 1:18 the 2500 ft/min is about what A320 does at cruise speed with engines at idle. So more than 3000 ft/min means spoilers and that is wasting energy. –  Jan Hudec Aug 22 at 21:43
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@Terry: yep, that could be a big problem over Texas! :-) –  Bob Jarvis Aug 23 at 2:32
    
@BobJarvis But not in Austin, which is full of hippie-liberals! –  David Richerby Aug 25 at 18:37

Normal descent rates will vary by operator and by airframe, but in general they follow this logic:

  1. Power at flight idle,
  2. Airspeed at or near max mach number or max KIAS,
  3. Spoilers stowed.

Generally they wont push airspeed right up to the limits and incorporate a small margin, but this is general idea. This is the most efficient descent because it means you stayed as high as long as possible but not so long you need to "cheat" with lift-destroying devices. Whatever descent rate this configuration results in is the "normal" rate.

The actual descent rate will vary in practice. ATC might descend you early, and then you will use a lower descent rate to stay higher longer (500 fpm unless they ask for more). If they keep you high too long, you opt for a higher rate of descent (spoilers!). The descent rate chosen will also be effected by the vertical wind profile. You may have a strong headwind and since you are covering ground slower opt for a slower descent, but if that headwind reduces in strength that might end up biting you later. Many descents are "at pilot discretion", and thus we'll typically stay high as long as we can until we can descend at (or near) idle to meet our crossing restrictions.

Passenger comfort isn't directly correlated to descent rate. Many pax will claim their ears popped more, but in reality the cabin pressure change is the same regardless of the descent rate. Things the pax are sensitive to that do occur are accelerations, and how fast the airplane is pitched into the descent will have a large impact on how the descent rate is perceived in the cabin (This is the same reason a vertical deviation of 50 feet in turbulence makes passengers think they dropped 1000 ft, it is all in the acceleration). Deck angle may also play a role in passenger perception.

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Given the noise abatement initiatives at AUS, it could be a voluntary procedure, usable depending on conditions. (Steep ascents are sometimes used for this reason at airports with noise issues.)

Looking at the data, the steep descent rate (3960, 3840 fpm) was only for a couple of minutes near the start of the descent. True, next day's flight didn't exceed 3000 fpm, but the previous day's got to 3660 fpm for a couple of minutes, so your flight doesn't look way out of line. In fact, the odd feature of your flight seems rather to have been the extremely slow ascent, almost half the typical climb rate.

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First of all, don't take the altitudes and descent/climb rates as fact from FlightAware, they could be pulling from different sources with different time delays or time keeping.

Reasons for higher rate of descent at the initial part of descent, could be anything from a late handoff from a previous center sector, to restricted/military areas being active, or having to wait til late to start a descent.

Normal decent rate varies... but I'd say an average of around 2000-2500 feet per minute.

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I was on the flight, and it was a very steep descent. Steeper than anything I've experienced before. I only checked FlightAware because it felt a lot steeper than usual. –  Philippe Leybaert Aug 23 at 4:23

3000 fpm is considered normal at one company that I worked for. Obviously, we can deviate within reason. I have routinely descended at more than 3000fpm in non-emergency situations. Aggressive stepdowns on a STAR, controller requests expedited descent for traffic, slam dunk from 10,000ft+ (cleared for visual, hand off to tower, immediately cleared to land), etc.

That being said, 3000fpm was set as normal as per GOM for a reason; passenger comfort.

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