SuseJ772
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744 (and Others) FPM Descent Range At Touchdown

Sun Feb 11, 2007 6:34 am

This may have been brought up, but I would be really curious as to what is the descent rate range at touchdown acceptable on a 744 (i.e. what is ideal to the maximum rate before damage to the landing gear).

I always thought touchdown was suppose to be 700 fpm, but I was talking with someone and they told me they thought 700 fpm was the descent rate from intercepting the glidescope until flare, but at actual touchdown you want to be going about 300 fpm?

I'd also be curious to know other aircraft types as well and how/why it differs between models.

P.S. I found this article Vertical Speed At Touchdown..., but it didn't answer the range, but only talked about the ideal VS
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chksix
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RE: 744 (and Others) FPM Descent Range At Touchdown

Sun Feb 11, 2007 9:49 am

The landing gear is certified by dropping it mounted in a rig from 30 ft at max weight and max speed. That would be similar to a stall and drop to the runway just after takeoff.

They showed the A380 MLG being tested in this way on tv a while ago.
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flyf15
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RE: 744 (and Others) FPM Descent Range At Touchdown

Sun Feb 11, 2007 10:32 am

In the CRJ, typical touchdowns are in the 200-300 fpm range. Really depends on the specific landing, pilot technique, etc. At my airline, aything over 600fpm requires a hard landing inspection prior to its next flight.
 
3DPlanes
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RE: 744 (and Others) FPM Descent Range At Touchdown

Sun Feb 11, 2007 12:21 pm

Quoting Chksix (Reply 1):
dropping it mounted in a rig from 30 ft

If my mind isn't failing me, d = 1/2at^2, and v=at

Solving for t, gives 1.37 seconds to go 30 ft. With v at impact equal to 2630 feet per minute.

Given those numbers (and assuming my math is right) - it almost seems conservative to check at a value as low as 600 fpm...
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SuseJ772
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RE: 744 (and Others) FPM Descent Range At Touchdown

Sun Feb 11, 2007 2:15 pm

Quoting 3DPlanes (Reply 3):
Solving for t, gives 1.37 seconds to go 30 ft. With v at impact equal to 2630 feet per minute.

I am not sure what all the variables stand for (d, a, t) but your math is quite interesting. I am wondering now from the pilots on the forum what their "hardest" landing they can remember was?

Also, I am curious as to what ranges of hard landings pilots have had that after check have yielded no problems and what has yielded problems - or where was the threshold that on their type seems to produce a problem in the real world (not just mathematically speaking).
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PhilSquares
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RE: 744 (and Others) FPM Descent Range At Touchdown

Sun Feb 11, 2007 3:14 pm

On the 744, 600 FPM is the value that will satisfy the on board monitoring system. If it's above that then there's an "event" that will trigger an analysis of the ACMS function of the CMC.

Generally, the touchdown VVI you have at touchdown is in the vicinity of 200-300 FPM. Touchdown in that range and you will have a comfortable touchdown within the required touchdown area on the runway.
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chksix
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RE: 744 (and Others) FPM Descent Range At Touchdown

Sun Feb 11, 2007 5:18 pm

I have to add here that the 30 ft height was a guess, it may have been higher. The gear assembly is so big on that plane that it makes judging scale difficult  Smile
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Tristarsteve
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RE: 744 (and Others) FPM Descent Range At Touchdown

Sun Feb 11, 2007 9:42 pm

A little storey here about what can go wrong.

Many years ago, the Royal Air Force decided to try out the autoland on their newly acquired Tristar.. They had not been trained on autoland, but found out later that the tech log entry stating that autoland was not available was not just a training issue, but backed up by several pulled and collared circuit breakers. The collision with the ground threw the mighty L1011 back into the air, and also broke the wing main spar and punctured the fuel tanks in many places. Five tonnes of fuel was lost during the following circuit and manual landing.
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cedarjet
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RE: 744 (and Others) FPM Descent Range At Touchdown

Mon Feb 12, 2007 1:58 am

Quoting TristarSteve (Reply 7):
Many years ago, the Royal Air Force decided to try out the autoland on their newly acquired Tristar.. They had not been trained on autoland, but found out later that the tech log entry stating that autoland was not available was not just a training issue, but backed up by several pulled and collared circuit breakers. The collision with the ground threw the mighty L1011 back into the air, and also broke the wing main spar and punctured the fuel tanks in many places. Five tonnes of fuel was lost during the following circuit and manual landing.

So they just let it fall onto the runway from circuit height, thinking it was being flown by the computer?! Oh man. Government work. What can you say?!
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citationjet
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RE: 744 (and Others) FPM Descent Range At Touchdown

Mon Feb 12, 2007 2:36 am

Landing gear on commercial aircraft certified to Part 25 criteria. The landing gear limit (once in a lifetime) load is 10 feet per second at the maximum landing weight and 6 feet per second at the maximum takeoff weight per FAR 25.473. In addition the gear is tested to a reserve energy requirement (ultimate load case) of 12 feet per second. At 12 feet per second at max landing weight, the tires can blow, and the structure can permanently deform, but the landing gear cannot break.
Boeing Flown: 701,702,703;717;720;721,722;731,732,733,734,735,737,738,739;741,742,743,744,747SP;752,753;762,763;772,773.
 
474218
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RE: 744 (and Others) FPM Descent Range At Touchdown

Mon Feb 12, 2007 3:09 am

Quoting CitationJet (Reply 9):
Landing gear on commercial aircraft certified to Part 25 criteria. The landing gear limit (once in a lifetime) load is 10 feet per second at the maximum landing weight and 6 feet per second at the maximum takeoff weight per FAR 25.473. In addition the gear is tested to a reserve energy requirement (ultimate load case) of 12 feet per second. At 12 feet per second at max landing weight, the tires can blow, and the structure can permanently deform, but the landing gear cannot break.

Citation Jet is correct. 6 feet per second (300 feet per minute) is the desired sink rate for landing. I was involved with the RAF incident TriStar Steve described (in Reply 7) and the sink rate in that incident was calculated as being about 18 feet per second.
 
FredT
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RE: 744 (and Others) FPM Descent Range At Touchdown

Mon Feb 12, 2007 4:09 am

Quoting Cedarjet (Reply 8):
So they just let it fall onto the runway from circuit height, thinking it was being flown by the computer?! Oh man. Government work. What can you say?!

Squawk: "Autoland touchdown rough"
Writeoff: "Autoland not installed on this aircraft"
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citationjet
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RE: 744 (and Others) FPM Descent Range At Touchdown

Mon Feb 12, 2007 4:45 am

During certification testing of the MD-80 landing distance performance testing was conducted. Due to a high descent rate landing, the vertical tail separated from the aircraft. The aircraft was heavily instrumented, therefore much information was known about the performance parameters. The aircraft landed 2,500 lb above the max landing weight at 125 KIAS, with a descent rate of 990 feet per minute (16.5 fps) based on the on board instrumentation on the experimental aircraft (page 6 of NTSB report).


Here is what a 16.5 ft per second landing looks like!!!!!!!!!
Link to video of landing:
http://www.youtube.com/results?searc...y=dc-9+crash+landing&search=Search

Link to NTSB report of the accident:
http://www.alexisparkinn.com/Photoga...lery2/MD80_NTSBReport/AAR82-02.pdf

.

[Edited 2007-02-11 20:51:48]

[Edited 2007-02-11 21:04:32]
Boeing Flown: 701,702,703;717;720;721,722;731,732,733,734,735,737,738,739;741,742,743,744,747SP;752,753;762,763;772,773.
 
SuseJ772
Topic Author
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RE: 744 (and Others) FPM Descent Range At Touchdown

Mon Feb 12, 2007 1:31 pm

Quoting PhilSquares (Reply 5):
On the 744, 600 FPM is the value that will satisfy the on board monitoring system.


Knowing that cracks me up now that I realize the error in my FlightSim practice. I had always been shooting for a 700 FPM at touchdown, and then obviously would have harder landings from time to time at about 1300-1500 FPM. I'll have to work on my flare  wink 
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BAE146QT
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RE: 744 (and Others) FPM Descent Range At Touchdown

Mon Feb 12, 2007 6:47 pm

I may be remembering this incorrectly, but I'm sure I read that for the 737 at least, it's possible to get the descent rate too low.

Something to do with the wheels not spinning up and therefore flat-spotting the tyres.

Quoting SuseJ772:
Knowing that cracks me up now that I realize the error in my FlightSim practice.

I don't know if you've ever tried FSPassengers, but a touchdown rate that high (700FPM) will earn you some choice words from your copilot and will require some expensive maint. The pax also complain, ("-200 points. Excessive descent rate. Passengers wondered if they had been shot down").
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FredT
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RE: 744 (and Others) FPM Descent Range At Touchdow

Tue Feb 13, 2007 3:31 am

Quoting BAe146QT (Reply 14):
I may be remembering this incorrectly, but I'm sure I read that for the 737 at least, it's possible to get the descent rate too low.

Something to do with the wheels not spinning up and therefore flat-spotting the tyres.

On a wet runway you want a firm touchdown. This will push the tyres through the water/slush on the runway and decrease the risk of aquaplaning. I doubt you could ever flat spot tires from skidding them along the runway without touching down. They'd spin up allright. The friction working on creating a flat spot is the same friction working on getting the tyre spinning.

Aquaplaning can cause rubber reversion though. This can damage the tyre, much like locking the wheel, and is probably the source behind what you heard.

Edit: An excessively smooth touchdown can delay the activation of the weight on wheels switch. This can, depending on the aircraft and the function of its systems, cause secondary effects.

Rgds,
/Fred

[Edited 2007-02-12 19:32:27]
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BAE146QT
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RE: 744 (and Others) FPM Descent Range At Touchdown

Tue Feb 13, 2007 4:17 am

Thank you Fred. Probably more due to my faulty memory than anything else.
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BAE146QT
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RE: 744 (and Others) FPM Descent Range At Touchdown

Wed Feb 14, 2007 6:21 pm

Incidentally, I just came across this;

[begin fair use]

Quote:
Unfortunately the 737-700 was particularly prone to a dramatic shudder from the main landing gear if you tried to land smoothly. Fortunately Boeing started fitting shimmy dampers to this series from L/N 406 (Nov 1999) and a retrofit was made available.

[/fair use]
(from http://www.b737.org.uk/landinggear.htm)

Obviously specific to a particular airframe, but I thought it interesting that there is another reason why you might want a firm touchdown.


P.s. Don't know if the owner of that site posts here, but it's brilliant - thank you.
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prok
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RE: 744 (and Others) FPM Descent Range At Touchdown

Thu Feb 15, 2007 12:42 am

Quoting SuseJ772 (Thread starter):
someone and they told me they thought 700 fpm was the descent rate from intercepting the glidescope until flare

On the 744 the descent rate will be around 900 fpm on the glideslope.
 
BAE146QT
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RE: 744 (and Others) FPM Descent Range At Touchdown

Thu Feb 15, 2007 3:04 am

Quoting Prok:
On the 744 the descent rate will be around 900 fpm on the glideslope.

I think the problem was that Suse was confusing descent rate on approach with the rate of descent at touchdown.  Wink

And in post 14, I made the mistake of not explicitly differentiating either - but that is what I meant!
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speedracer1407
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RE: 744 (and Others) FPM Descent Range At Touchdown

Thu Feb 15, 2007 2:15 pm

Quoting FredT (Reply 15):
Edit: An excessively smooth touchdown can delay the activation of the weight on wheels switch. This can, depending on the aircraft and the function of its systems, cause secondary effects.

We all know it's hard identify the causes of unfamiliar situations while onboard as a passenger, but I feel like this may have happened on the last flight I took from ORD to my connection in IAH.

During the flare, the pilot seemed to arrest the sinkrate completely. I had a good look at the wings, and the pilot had the ailerons flapping up and down while we floated for quite a while. I'm sure I felt the gear touch the ground EVER so lightly a good few seconds before the spoilers deployed. Normally, and on all three other legs of my roundtrip flight, we touched firmly, and the spoilers deployed upon the instant of touchdown.

As an aside, this landing was amusing to me because of how heavy handed the pilot seemed to handle the plane (understandably) druing our taxi and takeoff from ORD during the snowstorm on tuesday afternoon.
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BAE146QT
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RE: 744 (and Others) FPM Descent Range At Touchdown

Thu Feb 15, 2007 5:46 pm

Quoting Speedracer1407:
I had a good look at the wings, and the pilot had the ailerons flapping up and down while we floated for quite a while.

I had a similar experience on a 757 landing at Lanzarote. My perception was that the pilot was "feeling for the runway" with the left main gear. On reflection, it was probably just crosswinds.
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