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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 '14 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 '14 at 18:41
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The old slam-dunk approach perhaps? – falstro Aug 22 '14 at 18:56
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In the EMB-145 I flew SOP was 3000 ft/min though under certain conditions 4000 ft/min was fine. – casey Aug 23 '14 at 2:24
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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 '14 at 15:32
up vote 12 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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11  
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 '14 at 19:19
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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 '14 at 21:43
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@Terry: yep, that could be a big problem over Texas! :-) – Bob Jarvis Aug 23 '14 at 2:32
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@BobJarvis But not in Austin, which is full of hippie-liberals! – David Richerby Aug 25 '14 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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1  
And since it's only for a couple of minutes, it's possible that ATC just gave a fairly late call "Pass SOMEVOR at or below FL 250" a little later than expected could mean the pilot has to dump the spoilers and point the nose down to meet that requirement. – Jon Story Mar 21 at 14:46

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 '14 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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Yeah the latter point here is about right: 3000ft is at the "high" (or low, depending on your perspective...) end of comfortable descent rates. That doesn't mean it will never be busted as a soft limit, but a little over this for a few minutes isn't entirely unreasonable. – Jon Story Mar 21 at 14:48

There is no single normal descent rate in fpm. It varies widely. A normal descent clearance only requires at least 500fpm. Unless ATC gives a hurry/expedite command. For most of the explanation I assume an idle thrust descent, without an emergency or any reason to waste energy on purpose. A normal descent is almost a glide, with the engines producing minimum idle thrust.

In that scenario, descent rate is largely proportional to TAS (true airspeed). So when the descent begins, say from a Mach 0.78 (250kias/450ktas) cruise, descent rate should be around 2500fpm. This rate of descent should be maintained until transition from Mach to IAS happens. At that point the lower the aircraft goes, the slower the TAS will be, as a consequence, the descent rate will slow down proportionately. At the same time, idle thrust at high altitude is quite low. The lower the aircraft goes, the higher idle thrust will be, further moderating descent rate. In many jurisdictions there's a 250kias limit, so the descent rate should reduce a lot 11500 and 10000 to slow down from around 280kias to 250kias, since the true airspeed reduced, so will the descent rate. Assuming no further speed restrictions, pilot should keep 240-250kias (resulting in about 1300fpm down) until entering the base leg or in the case of a straight in approach around 15nm final. At that point the aircraft should be gear down and rapidly transition to final approach flaps and final approach speed, again, true air speed reduces again, so does rate of descent, assuming a 140kias final, this should lead to around 700fpm descent. A 3000fpm descent could happen if ATC assigns an altitude restriction, and the pilot trade airspeed for altitude in idle/low thrust, until a minimum safe speed is achieved. Once ATC clears the aircraft to continue its descent, the pilot can trade altitude for airspeed which would result in higher sink rate for a few minutes until the aircraft accelerates back to cruise descent. If the pilot is too high or is given a hurry through descent clearance, it can deploy spoilers and/or lower the landing gear which could result in a 4000fpm descent. Some congested airspace calls for quick descents to avoid invading climb/descent areas for other airports. There's this saying that speed breaks is for pilot's mistakes, but not for ATC's mistakes... That's not a good wisdom. If you don't cooperate with ATC, you might find yourself in a hold or unnecessary detour for your lack of compliance. What might look like a mistake for a pilot might be unavoidable for ATC (for instance short staffed for a few minutes).

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"At least 500fpm" is incorrect. U.S. ATC expects at least 1000'/minute descent rate. Some of the other numbers & explanations are incorrect as well. Down-voted for inaccuracies. – Ralph J Mar 20 at 19:31
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I would think ATC at a busy airport would want to kill a pilot who insists on final approach speed and flaps 15 miles out. – Zach Lipton Mar 20 at 19:46
    
I don't necessarily follow all the math and other items necessary to understand the difference between IAS and TAS, but 250kias/450ktas seems like a huge gap. Is that really reasonable? – FreeMan Mar 21 at 12:59
    
ATC might EXPECT 1000fpm, but the regulation is 500fpm. I watch planes for hours everyday on flightradar24 and see 500fpm descents all the time, although it really isn't the norm. If ATC asks you to descent just 2000ft, a 1000fpm descent would be unnecessary. You get 250kias for 450ktas flying quite high, while at sea level 250kias=250ktas. – Marcelo Pacheco Mar 21 at 13:02
    
Zach quite often traffic will be slowed down to 180kias by ATC on final, otherwise they would need too much spacing between birds on final. Just look at ORD / ATL on very busy hours. If an airline pilot wants final approach speed 10 miles out, ATC is the least of his concerns, his employer might fire him for wasting fuel on purpose. Its really interesting watching some 100% optimal descents where a B787 will go from FL400 to landing with speed really high until FL100, and still 250KIAS until final, really cool ! – Marcelo Pacheco Mar 21 at 14:25

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