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planecane
Topic Author
Posts: 1925
Joined: Thu Feb 09, 2017 4:58 pm

How common is 3800-4200 fpm descent?

Mon Feb 07, 2022 1:03 pm

On my flight this past Saturday it seemed our descent on approach was more rapid than I normally experience (I fly at least 7 round trips a year, usually more). Just before the announcement for the flight attendants to prepare for landing, I had 1/3 of a cup of water on my tray table and, I guess due to pitching down sharply, the cup slid right off the tray table and dumped water on my feet. I've never had anything like that happen unless there was turbulence. Even then, I've never had something just go flying off the tray table like that.

Spoilers were deployed for quite a while. They were definitely in use when the flaps were set and I don't think they were stowed until after the gear was down. I went on flightaware and, according to the log, there were 3 minutes of a 3800-4200 fpm descent rate (https://flightaware.com/live/flight/AAL2870/history/20220205/1909Z/KEGE/KMIA/tracklog). I look at the log of a lot of my flights and I've never noticed that high of a rate. It's possible that it happens but I noticed this time due to the water sliding off of the tray table.

I'm curious if this descent rate was out of the ordinary and if it was if there is an explanation as to why it was done. I know we flew pretty far south of MIA to go around a storm cell but I would think if anything that would allow the rate of descent to be slower because of the additional track miles.
 
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Starlionblue
Posts: 21175
Joined: Fri Feb 27, 2004 9:54 pm

Re: How common is 3800-4200 fpm descent?

Mon Feb 07, 2022 1:15 pm

It happens. Not really unusual if you get some track shortening. But it can happen in any case. I've had 5500fpm on the descent. Not really an issue. Sometimes you just need to get down in a hurry.
 
Flow2706
Posts: 343
Joined: Sat Jan 28, 2017 7:20 pm

Re: How common is 3800-4200 fpm descent?

Mon Feb 07, 2022 2:47 pm

Normal descend rates at normal speeds without speed brakes are around 2500 (high level) to around 1500/1800fpm (below around FL100), but for profile adjustments rates up to 6000/6500 are achievable in normal operations. Could be due to unexpected track shortening, last minute runway change etc.
 
Woodreau
Posts: 2315
Joined: Mon Sep 24, 2001 6:44 am

Re: How common is 3800-4200 fpm descent?

Mon Feb 07, 2022 4:47 pm

A descent like that is normal during a Mach/speed transition. The aircraft has to “accelerate” in the transition from indicated Mach to indicated airspeed, depending on the speed that is programmed in the FMS for the transition. In an idle descent, you’ll have to increase the descent rate to reach the profile airspeed.

It’s more common with a .83/335 descent profile and not as common with a .70/250 descent profile.

Your “high” descent rates for those 3 minutes occurred during the Mach/speed transition in the mid-20s. The call for flight attendants to prepare for landing occurs at 10000ft, so the descent rate when your water spilled wasn’t during the 4000-4200ft/min descent phase.
 
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zeke
Posts: 17397
Joined: Thu Dec 14, 2006 1:42 pm

Re: How common is 3800-4200 fpm descent?

Mon Feb 07, 2022 8:46 pm

planecane wrote:
On my flight this past Saturday it seemed our descent on approach was more rapid than I normally experience (I fly at least 7 round trips a year, usually more). Just before the announcement for the flight attendants to prepare for landing, I had 1/3 of a cup of water on my tray table and, I guess due to pitching down sharply, the cup slid right off the tray table and dumped water on my feet. I've never had anything like that happen unless there was turbulence. Even then, I've never had something just go flying off the tray table like that.

Spoilers were deployed for quite a while. They were definitely in use when the flaps were set and I don't think they were stowed until after the gear was down. I went on flightaware and, according to the log, there were 3 minutes of a 3800-4200 fpm descent rate (https://flightaware.com/live/flight/AAL2870/history/20220205/1909Z/KEGE/KMIA/tracklog). I look at the log of a lot of my flights and I've never noticed that high of a rate. It's possible that it happens but I noticed this time due to the water sliding off of the tray table.

I'm curious if this descent rate was out of the ordinary and if it was if there is an explanation as to why it was done. I know we flew pretty far south of MIA to go around a storm cell but I would think if anything that would allow the rate of descent to be slower because of the additional track miles.


Looking at the log they were descending fairly normally to the south east then they got a turn towards the airport when the ROD increased. This was probably the pilots having good situational awareness to recognise they are being cut in towards the runway and the track mikes are being reduced. Spoilers are far more effective at higher speed, if you need to make a correction for the profile do it early, they did it at the best time.

I cannot account for your glass falling off, the acceleration rates involved are very small.

At my home airport it is not uncommon to pass a waypoint near the airport at 19-21,000 ft, it is not unusual to have full speed brake and 300 kts dropping at 6000 fpm, speed brake is kept out approaching 10,000 ft to reduce to 250 kts.
 
planecane
Topic Author
Posts: 1925
Joined: Thu Feb 09, 2017 4:58 pm

Re: How common is 3800-4200 fpm descent?

Tue Feb 08, 2022 1:19 am

zeke wrote:

I cannot account for your glass falling off, the acceleration rates involved are very small.

At my home airport it is not uncommon to pass a waypoint near the airport at 19-21,000 ft, it is not unusual to have full speed brake and 300 kts dropping at 6000 fpm, speed brake is kept out approaching 10,000 ft to reduce to 250 kts.


Is it possible that there was enough of a pitch down to cause the drink to slide off? I was in row 24 (out of 27) so I was pretty far back.
 
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Starlionblue
Posts: 21175
Joined: Fri Feb 27, 2004 9:54 pm

Re: How common is 3800-4200 fpm descent?

Tue Feb 08, 2022 1:26 am

Some of those tray tables are slippery to be honest.
 
adipasqu
Posts: 265
Joined: Wed Apr 21, 2004 1:37 pm

Re: How common is 3800-4200 fpm descent?

Tue Feb 08, 2022 1:41 am

planecane wrote:

Is it possible that there was enough of a pitch down to cause the drink to slide off? I was in row 24 (out of 27) so I was pretty far back.


Of course there is. However, unless the aircraft's fuselage was bent, the pitch of the plane in row 24 will always be the same as in row 1. You might feel the pitch change more the further away you are from the wings (further from the pivot point), but I don't know if that would translate to more force on the cup on your tray table relative to over the wing.
 
bluecrew
Posts: 387
Joined: Sun Apr 20, 2014 3:13 am

Re: How common is 3800-4200 fpm descent?

Tue Feb 08, 2022 8:56 am

planecane wrote:
On my flight this past Saturday it seemed our descent on approach was more rapid than I normally experience (I fly at least 7 round trips a year, usually more). Just before the announcement for the flight attendants to prepare for landing, I had 1/3 of a cup of water on my tray table and, I guess due to pitching down sharply, the cup slid right off the tray table and dumped water on my feet. I've never had anything like that happen unless there was turbulence. Even then, I've never had something just go flying off the tray table like that.

Spoilers were deployed for quite a while. They were definitely in use when the flaps were set and I don't think they were stowed until after the gear was down. I went on flightaware and, according to the log, there were 3 minutes of a 3800-4200 fpm descent rate (https://flightaware.com/live/flight/AAL2870/history/20220205/1909Z/KEGE/KMIA/tracklog). I look at the log of a lot of my flights and I've never noticed that high of a rate. It's possible that it happens but I noticed this time due to the water sliding off of the tray table.

I'm curious if this descent rate was out of the ordinary and if it was if there is an explanation as to why it was done. I know we flew pretty far south of MIA to go around a storm cell but I would think if anything that would allow the rate of descent to be slower because of the additional track miles.

Exceedingly common if you have an effective speedbrake and fly anywhere they give you shortcuts.
Down to 11k but now you're all of a sudden direct the airport with 20 track miles to go? Yes please, let's get there and drop anything that produces drag.

Flightaware and even ADS-B Exchange data is notoriously off the mark here, it will show absolutely wacky values that don't reflect what actually happened. Great tool, and very cool that this content is out there, but it's not perfect or audited.
 
planecane
Topic Author
Posts: 1925
Joined: Thu Feb 09, 2017 4:58 pm

Re: How common is 3800-4200 fpm descent?

Tue Feb 08, 2022 12:57 pm

adipasqu wrote:
planecane wrote:

Is it possible that there was enough of a pitch down to cause the drink to slide off? I was in row 24 (out of 27) so I was pretty far back.


Of course there is. However, unless the aircraft's fuselage was bent, the pitch of the plane in row 24 will always be the same as in row 1. You might feel the pitch change more the further away you are from the wings (further from the pivot point), but I don't know if that would translate to more force on the cup on your tray table relative to over the wing.


Good point. I was thinking that in the back I'd be higher than the in the center but it slipped my mind that it's still the same angle.

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