Vertical speed at touchdown
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Vertical speed at touchdown
I am into flight simulators and have always wondered what would be a landing which would most definitely end as a crash, on the average airliner?
I have seen some people on a simulator with a vertical speed at touchdown of -1,000 ft/min. What would be good cutoff point to say that anything above that reading, should be considered a crash?
I have seen some people on a simulator with a vertical speed at touchdown of -1,000 ft/min. What would be good cutoff point to say that anything above that reading, should be considered a crash?
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The design criteria for a landing gear assumes 10 fps impact at max landing weight for the "design" case.
While impacts in excess of that (i.e. >600 fpm) may not be a "crash" most OEMs would consider that a "hard landing" threshold so making that a "fail" criteria wouldn't seem unreasonable if one were seeking a simple pass/fail threshold.
While impacts in excess of that (i.e. >600 fpm) may not be a "crash" most OEMs would consider that a "hard landing" threshold so making that a "fail" criteria wouldn't seem unreasonable if one were seeking a simple pass/fail threshold.
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Is there anything in modern transport aircraft that records peak accelerations for various parts of the aircraft, in order to flag the aircraft for inspection or other action if certain design limits are exceeded? I'm thinking mainly of hard landings or extreme turbulence. Clearly, pilots have a good feel for what's not a problem and what might be a problem, but a documented record would make it easier to draw the line unambiguously.
Perhaps those extra boxes that the airlines use in addition to black boxes (I can't remember their names) might record this type of information? Also, if they do record the information, and if it is archived and integrated over time, it would be possible to calculate total stress and fatigue risk for parts of the airframe over its lifetime, perhaps avoiding unnecessary maintenance and also forcing needed maintenance if an aircraft has fortuitously had a large number of rough rides.
Perhaps those extra boxes that the airlines use in addition to black boxes (I can't remember their names) might record this type of information? Also, if they do record the information, and if it is archived and integrated over time, it would be possible to calculate total stress and fatigue risk for parts of the airframe over its lifetime, perhaps avoiding unnecessary maintenance and also forcing needed maintenance if an aircraft has fortuitously had a large number of rough rides.
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Mad Flt Scientist
Are you specifying 10 fps as an ultimate design number or with safety factor included.?
In my day at Boscombe Down I seem to recall 7 feet per second and beyond as a heavy landing requiring inspection. For Naval aircraft landing without a flare it was considerably more. Cannot recall the criteria for considering side loads.
Presumably the specifications for tyre designs line up with or are more stringent than for the gears and there are different numbers for nose gears.
There will be precious few pilot who can judge how close they come to, or exceed, the safe sink rate and some gears are much kinder than others having built in articulated damping. Ground effect helps a bit. I'd be surprised if the modern simulators provide an accurate feel of a landing at the limit. Any comments?
Are you specifying 10 fps as an ultimate design number or with safety factor included.?
In my day at Boscombe Down I seem to recall 7 feet per second and beyond as a heavy landing requiring inspection. For Naval aircraft landing without a flare it was considerably more. Cannot recall the criteria for considering side loads.
Presumably the specifications for tyre designs line up with or are more stringent than for the gears and there are different numbers for nose gears.
There will be precious few pilot who can judge how close they come to, or exceed, the safe sink rate and some gears are much kinder than others having built in articulated damping. Ground effect helps a bit. I'd be surprised if the modern simulators provide an accurate feel of a landing at the limit. Any comments?
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The A330 will print a Post Flight Report obviously at the end of the flight
.
One endeavours to avoid a Load 15 report but the following info comes from my OM Part A:
There are several categories of hard landing and this will dictate the level of post flight inspection required.
The criteria and parameters for aircraft type are as follows.
1. A hard landing is a landing with an aircraft weight less than the Maximum Landing Weight (MLW) and:-
• a vertical acceleration (Vert G) equal to or more than 2.6g and less than 2.86g at aircraft Centre of Gravity (CG) or,
• a vertical speed (Vs) equal to or more than 10 ft/sec and less than 14 ft/sec.
2. A severe hard landing is a landing with an aircraft weight less than the Maximum Landing Weight (MLW) and :-
• a vertical acceleration (Vert G) equal to or more than 2.86g at aircraft Centre of Gravity (CG) or,
• a vertical speed (Vs) equal to or more than 14 ft/sec.
. One endeavours to avoid a Load 15 report but the following info comes from my OM Part A:
There are several categories of hard landing and this will dictate the level of post flight inspection required.
The criteria and parameters for aircraft type are as follows.
1. A hard landing is a landing with an aircraft weight less than the Maximum Landing Weight (MLW) and:-
• a vertical acceleration (Vert G) equal to or more than 2.6g and less than 2.86g at aircraft Centre of Gravity (CG) or,
• a vertical speed (Vs) equal to or more than 10 ft/sec and less than 14 ft/sec.
2. A severe hard landing is a landing with an aircraft weight less than the Maximum Landing Weight (MLW) and :-
• a vertical acceleration (Vert G) equal to or more than 2.86g at aircraft Centre of Gravity (CG) or,
• a vertical speed (Vs) equal to or more than 14 ft/sec.
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Milt;
From FAR 25, the 10 fps figure is used to derive transport category aircraft limit loads; the condition has to be combined with all weight/cg conditions up to MLW. In addition, you have to consider 6 fps for weight/cg between MLW and MTOW. Ultimate loads will typically be 1.5 the limit loads. There is also, for the landing gear itself, the reserve energy condition of 12 fps; this is roughly equivalent to the 1.5 factor ( 12/10 squared = 1.44)
Are you specifying 10 fps as an ultimate design number or with safety factor included.?
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Thanks for the answers guys!! Very interesting reading nonetheless... I gues from -600 ft/min to -840 ft/min would be a hard landing and anything worse than -840 ft/min would be a severe hard landing. At least this could be used as a global threshold.
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In this context, what I have trouble with is the use of vertical acceleration as a yardstick, with a two decimal points resolution no less ....
I have no problem with using vertical speed, even at only one or two samples per second.... The whole event is pretty well defined by the vertical speed at the last few seconds (think inertia), and the geometry/characteristics of the landing gear.
Does somebody here have figures for the sample rates for vertical acceleration in a typical FDR or QAR?
Since at 10 fps the whole touchdown event is over in about a second, I would have thought you'd need something like 10 to 20 samples/sec to get an accuracy of even a few percent for peak G, which seems to be a waste of bandwidth and recording capacity.
Or am I missing something and is there a separate peak-reading device?
I have no problem with using vertical speed, even at only one or two samples per second.... The whole event is pretty well defined by the vertical speed at the last few seconds (think inertia), and the geometry/characteristics of the landing gear.
Does somebody here have figures for the sample rates for vertical acceleration in a typical FDR or QAR?
Since at 10 fps the whole touchdown event is over in about a second, I would have thought you'd need something like 10 to 20 samples/sec to get an accuracy of even a few percent for peak G, which seems to be a waste of bandwidth and recording capacity.
Or am I missing something and is there a separate peak-reading device?
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The required sampling rate for vertical accel on FDRs is 8 Hz. The actual sampling rate may vary from box to box. For sure, 8Hz is not enough to pick out the true peak 'g' from a hard landing. The load peak from a landing is typically within a quarter of a second from initial contact between wheel and runway surface. But it depends the descent rate... high descent rate landings peak sooner, low descent rate landings peak later. The landing weight and gear type will also make a difference.
In addition, you may also run into filtering problems, as a lot of dedicated FDR accelerometers that I know of have a roll-off around 4 Hz, and it's a rather sharp roll-off. If you have a very high descent rate landing, say above 14 - 16 fps, the frequency content of the load peak may be attenuated significantly.
Using a vertical-speed indicator sampled at only once per second is ill-advised, as the vertical speed will vary a lot in the last seconds before impact, especially in those conditions which are likely to produce hard landings (i.e. steep approach, gusty conditions, etc).
Remember that the may transport category aircraft are certified, what counts is the vertical descent rate at the moment that the tire first makes contact with the runway (of course, certification is in a perfect world, so we assume no bounce after initial contact).
In addition, you may also run into filtering problems, as a lot of dedicated FDR accelerometers that I know of have a roll-off around 4 Hz, and it's a rather sharp roll-off. If you have a very high descent rate landing, say above 14 - 16 fps, the frequency content of the load peak may be attenuated significantly.
Using a vertical-speed indicator sampled at only once per second is ill-advised, as the vertical speed will vary a lot in the last seconds before impact, especially in those conditions which are likely to produce hard landings (i.e. steep approach, gusty conditions, etc).
Remember that the may transport category aircraft are certified, what counts is the vertical descent rate at the moment that the tire first makes contact with the runway (of course, certification is in a perfect world, so we assume no bounce after initial contact).
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DFDAU
In my airline we use Digital Flight Data Acuisition Units to record amongst other things fuel flows, engine health reports and any number of additonal (to the FDR) parameters that Engineering would like to see tracked.
The Flight Summary Report (auto printed on engine shutdown) includes Vetical G reports.
MAX VG for the entire flight
MIN VG for the entire flight
MAX VG LAND - the G on touchdown. If that figure is greater than 1.8G we have to call Maintrol and a heavy landing inspection will be carried out within 9 sectors.
However, whilst one cannot assume that we pilots are able to judge the subtleties of G on landing we DO generally know when "we've crunched her in" or "stroked her on". What is interesting is that the VG figures often quoted for a typical 'greaser' often is much higher than one would expect (1.2 -1.4G) and on the days where you have carried out the positive landing that Mr B loves so much you end up with a figure often lower (1.1 -1.3G).
I've never been told where these accelerometers are fitted and whether they are converted due to angular mountings but the data that the little green box give us is not treated as Gospel. If in doubt we would call the company to pull the Quick Access Recorder (different to the DFDAU) data to determine the actual touchdown ROD etc..
The Flight Summary Report (auto printed on engine shutdown) includes Vetical G reports.
MAX VG for the entire flight
MIN VG for the entire flight
MAX VG LAND - the G on touchdown. If that figure is greater than 1.8G we have to call Maintrol and a heavy landing inspection will be carried out within 9 sectors.
However, whilst one cannot assume that we pilots are able to judge the subtleties of G on landing we DO generally know when "we've crunched her in" or "stroked her on". What is interesting is that the VG figures often quoted for a typical 'greaser' often is much higher than one would expect (1.2 -1.4G) and on the days where you have carried out the positive landing that Mr B loves so much you end up with a figure often lower (1.1 -1.3G).
I've never been told where these accelerometers are fitted and whether they are converted due to angular mountings but the data that the little green box give us is not treated as Gospel. If in doubt we would call the company to pull the Quick Access Recorder (different to the DFDAU) data to determine the actual touchdown ROD etc..
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How can YOU tell whether the landings before the one you are going to make next were 'heavy' ? Does your aircraft type have a crush washer at full oleo stroke or whatever that you should look at instead of traditionally kicking a tyre during your walk around?
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From the 767 Maint Manual (747 similar)
The key words in the Manual are the first ones "If the Pilot determines the airplane had a hard landing, a structural inspection is necessary" So we work on the basis of, if it's written in the tech log as a hard landing we do the inspection. As a matter of course we pull and analyse the data recorders to get the g-level but if it happens to be below the guide line limit in the book we certainly don't cancel the inspection. Let's face it, tech crew generally aren't in a rush to write up in the log that they've had a hard landing - so when they do make that entry it's generally a real event.
World of Tweed has kindly backed me up,
At my airline, we take your word over the data stream(too unreliable and too many variables)
If the pilot determines the airplane had a hard landing, a
structural inspection is necessary.
structural inspection is necessary.
(1) For landing at or below the maximum design landing weight on
airplanes with flight data recording systems capable of at
least eight (8) samples per second, the following can be used:
An indication of a hard landing on the main landing gear is a
peak recorded vertical acceleration that exceeds 1.8 G
(incremental 0.8 G). This vertical accelerometer data must be
measured by the flight data recorder accelerometer at a data
sampling rate of at least eight (8) samples per second. This
vertical acceleration G-level threshold is valid for a
conventional landing with impact with no more than two (2)
degrees of airplane roll, main landing gear touchdown first and
normal rotation onto the nose gear.For a hard landing that is a
hard nose landing or is accompanied by more than two (2) degrees
of roll at the time of main landing gear impact, the recorded peak
acceleration can be significantly less than 1.8 G, but a hard landing
inspection may still be necessary.
airplanes with flight data recording systems capable of at
least eight (8) samples per second, the following can be used:
An indication of a hard landing on the main landing gear is a
peak recorded vertical acceleration that exceeds 1.8 G
(incremental 0.8 G). This vertical accelerometer data must be
measured by the flight data recorder accelerometer at a data
sampling rate of at least eight (8) samples per second. This
vertical acceleration G-level threshold is valid for a
conventional landing with impact with no more than two (2)
degrees of airplane roll, main landing gear touchdown first and
normal rotation onto the nose gear.For a hard landing that is a
hard nose landing or is accompanied by more than two (2) degrees
of roll at the time of main landing gear impact, the recorded peak
acceleration can be significantly less than 1.8 G, but a hard landing
inspection may still be necessary.
(2) For landing at or below the maximum design landing weight on
airplanes with flight data recording systems capable of at
least sixteen (16) samples per second, the following can be
used:
An indication of a hard landing on the main landing gear
is a peak recorded vertical acceleration that exceeds 1.9 G
(incremental 0.9 G). This vertical accelerometer data must be
measured by the flight data recorder accelerometer at a data
sampling rate of at least sixteen (16) samples per second.
This vertical acceleration G-level threshold is valid for a
conventional landing with impact with no more than two (2)
degrees of airplane roll, main landing gear touchdown first and
normal rotation onto the nose gear. For a hard landing that is
a hard nose landing or is accompanied by more than two (2)
degrees of roll at the time of main landing gear impact, the
recorded peak acceleration can be significantly less than 1.9
G, but a hard landing inspection may still be necessary.
airplanes with flight data recording systems capable of at
least sixteen (16) samples per second, the following can be
used:
An indication of a hard landing on the main landing gear
is a peak recorded vertical acceleration that exceeds 1.9 G
(incremental 0.9 G). This vertical accelerometer data must be
measured by the flight data recorder accelerometer at a data
sampling rate of at least sixteen (16) samples per second.
This vertical acceleration G-level threshold is valid for a
conventional landing with impact with no more than two (2)
degrees of airplane roll, main landing gear touchdown first and
normal rotation onto the nose gear. For a hard landing that is
a hard nose landing or is accompanied by more than two (2)
degrees of roll at the time of main landing gear impact, the
recorded peak acceleration can be significantly less than 1.9
G, but a hard landing inspection may still be necessary.
The key words in the Manual are the first ones "If the Pilot determines the airplane had a hard landing, a structural inspection is necessary" So we work on the basis of, if it's written in the tech log as a hard landing we do the inspection. As a matter of course we pull and analyse the data recorders to get the g-level but if it happens to be below the guide line limit in the book we certainly don't cancel the inspection. Let's face it, tech crew generally aren't in a rush to write up in the log that they've had a hard landing - so when they do make that entry it's generally a real event.
World of Tweed has kindly backed me up,
However, whilst one cannot assume that we pilots are able to judge the subtleties of G on landing we DO generally know when "we've crunched her in" or "stroked her on".
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I've never been told where these accelerometers are fitted
On a 747 it's in the right hand wing gear wheel well.
In the vertical axis they can generally measure between about -3g to +6g and in the lateral and longitudinal axis about -1g to +1g.
