"Push" recoveries
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I wonder what would happen if you omitted the 'push' from that sequence of events? Probably not much in terms of the firmness of the landing but maybe you are lessening the chances of a tailstrike by reducing/holding the attitude just prior to touchdown. Maybe that is what this was all about to begin with?
It feels akin to cushioning a bump when skiing so as not to get airborne - a quick bending of the knees at the right moment.
To me it seems obvious I'm simply rotating the tail and therefore the gear, up slightly (akin to the 'knee bend') and if I time it right, it produces a very soft touchdown. If I simply hold the flare attitude without this input, it produces a firmer (but a 'proper' Boeing) touchdown.
Can't explain it with maths, but I know I'm not imagining it. Come on you mathmaticians and aerodynamicists - let's see the formulae!
I used to fly the 737 and I don't remember ever doing that - maybe I should have!
The 'greasers' I got away with on the 73' came about in two different ways. One was to land on one wheel and the other was to close the thrust levers very slowly during the flare so you actually touched down with the thrust still reducing. Totally non-standard and reprehensible, of course.
When I sit on RW05 @LHR, waiting for my turn to cross
I often watch the landing traffic to spot technique.
I have seen some really good landings, i.e. ROD reduced to virtually zero just as the mainwheels kiss the tarmac, ruined by the speedbrakes shooting out and dropping the whole caboodle down the rest of the oleo travel.
Equally, I have watched what seemed like it was going to be a little too firm a touchdown, turn into a thing of beauty.
I think the trick is to get the mainwheels on the ground (gently, of course) but with a small ROD. You will then use up some of the oleo travel so when the speedbrakes deploy there isn't a crunch downwards.
Yes, it would seem that way but if you actually work out by how much the gear is moving, it comes down to inches. Unless there is something really weird going on in the ground effect. (Always possible...
)
The 'greasers' I got away with on the 73' came about in two different ways. One was to land on one wheel and the other was to close the thrust levers very slowly during the flare so you actually touched down with the thrust still reducing. Totally non-standard and reprehensible, of course.
When I sit on RW05 @LHR, waiting for my turn to cross
I often watch the landing traffic to spot technique.I have seen some really good landings, i.e. ROD reduced to virtually zero just as the mainwheels kiss the tarmac, ruined by the speedbrakes shooting out and dropping the whole caboodle down the rest of the oleo travel.
Equally, I have watched what seemed like it was going to be a little too firm a touchdown, turn into a thing of beauty.
I think the trick is to get the mainwheels on the ground (gently, of course) but with a small ROD. You will then use up some of the oleo travel so when the speedbrakes deploy there isn't a crunch downwards.
To me it seems obvious I'm simply rotating the tail and therefore the gear, up slightly (akin to the 'knee bend') and if I time it right, it produces a very soft touchdown. If I simply hold the flare attitude without this input, it produces a firmer (but a 'proper' Boeing) touchdown.
)
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I used to fly the 737 and I don't remember ever doing that - maybe I should have!
Therefore you remain unconvinced that my analysis of what's going on is correct.I need evidence from everyone familiar with this technique - speak up please!
I think the trick is to get the mainwheels on the ground (gently, of course) but with a small ROD. You will then use up some of the oleo travel so when the speedbrakes deploy there isn't a crunch downwards.
Using the 'Boeing push' as 411A so eloquently calls it, to a great extent eliminates that element of luck once mastered.
Come on Boeing pushers - help me out here........
Moderator
My recollection is that the check/roll technique was useful on the 722 if the last bit of the approach rolled belly up for some reason - generally due to the pilot's inexperience on type - a "save the day" way of reducing the wheel assembly descent rate at touchdown.
Having been taught that sort of technique - as a general way to land the old girl - I must confess I had not the slightest idea of what I was doing and not much idea of where the ground was during the exercise.
In frustration, I tried "normal" Cessna 172 landings and the problems went away... thereafter, I resolved to land an aircraft the way it worked .. without any smoke and mirrors. My two most memorable greasers were on the 722 ... during neither was there any shudder or sound associated with wheel spin-up. Actually quite unnerving ... all of us who flew the 200 knew what to expect if it stopped flying before the wheels were on the ground ...
Like one of the previous posters, I had the delightful pleasure of flying with a wonderful fellow (no longer with us, unfortunately) who would drive the 200 down to the aiming point .. and, just as my eyes opened to the size of dinner plates .. the aircraft would somehow stop going down and roll along the runway .. never could work out what he did to achieve that .. landing after landing. I suppose the umpteen thousands of hours he had on the model helped.
And then, the 100 was among the easiest of aircraft to land consistently well.
The 733 I found just about the easiest aircraft of any to land - one had to be completely ham-fisted to land that model badly.
Having been taught that sort of technique - as a general way to land the old girl - I must confess I had not the slightest idea of what I was doing and not much idea of where the ground was during the exercise.
In frustration, I tried "normal" Cessna 172 landings and the problems went away... thereafter, I resolved to land an aircraft the way it worked .. without any smoke and mirrors. My two most memorable greasers were on the 722 ... during neither was there any shudder or sound associated with wheel spin-up. Actually quite unnerving ... all of us who flew the 200 knew what to expect if it stopped flying before the wheels were on the ground ...
Like one of the previous posters, I had the delightful pleasure of flying with a wonderful fellow (no longer with us, unfortunately) who would drive the 200 down to the aiming point .. and, just as my eyes opened to the size of dinner plates .. the aircraft would somehow stop going down and roll along the runway .. never could work out what he did to achieve that .. landing after landing. I suppose the umpteen thousands of hours he had on the model helped.
And then, the 100 was among the easiest of aircraft to land consistently well.
The 733 I found just about the easiest aircraft of any to land - one had to be completely ham-fisted to land that model badly.
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John T, agreed that the B733 is on the whole very easy to land.
Without wanting to appear too boastful
, I must admit that with the 'push' I've found, much like the fellow you describe, that consistent greasers can be achieved again and again, sector after sector. I like to think my ROD to the flare is more normal though! (As you say, maybe the umpteen thousands of hours on the beast helps too.)
Coming back to FullWings and why this works, I think maybe you're discounting the effect of even a drop from say a foot in height onto the main wheels a little too much. With the ROD reduced to around 100 - 200 fpm in the flare, if we then hold the attitude, chances are we'll still get a noticeable jolt. (The correct technique of course I hasten to add). However, with the slight 'push' just before the mainwheels touch, we rotate the gear upwards just enough to reduce its ROD relative to the runway surface to just about zero. So the wheels kiss the tarmac, and interestingly often the aircraft sits down a liitle heavier on the oleos with this, but this is not noticeable to the pax in the same way as a firm landing. In fact, fairly easy to produce those 'are we down yet' type landings of which JT speaks.
An analogy would be it's like jumping up on the spot then making an exagerrated knee bend just as your feet touch the floor, followed by your thigh muscles (the oleos) needing to work harder to arrest the downward motion of your body.
Without wanting to appear too boastful
, I must admit that with the 'push' I've found, much like the fellow you describe, that consistent greasers can be achieved again and again, sector after sector. I like to think my ROD to the flare is more normal though! (As you say, maybe the umpteen thousands of hours on the beast helps too.)Coming back to FullWings and why this works, I think maybe you're discounting the effect of even a drop from say a foot in height onto the main wheels a little too much. With the ROD reduced to around 100 - 200 fpm in the flare, if we then hold the attitude, chances are we'll still get a noticeable jolt. (The correct technique of course I hasten to add). However, with the slight 'push' just before the mainwheels touch, we rotate the gear upwards just enough to reduce its ROD relative to the runway surface to just about zero. So the wheels kiss the tarmac, and interestingly often the aircraft sits down a liitle heavier on the oleos with this, but this is not noticeable to the pax in the same way as a firm landing. In fact, fairly easy to produce those 'are we down yet' type landings of which JT speaks.
An analogy would be it's like jumping up on the spot then making an exagerrated knee bend just as your feet touch the floor, followed by your thigh muscles (the oleos) needing to work harder to arrest the downward motion of your body.
This discussion reminds me of a (old) joke about proofs/correlation:
Guy gets into a compartment on the Brighton-London train and sits down. Opposite to him is a well-dressed gent reading the FT.
Soon after they leave the station, the gent mutters something under his breath and starts tearing small squares off his newspaper and throwing them out of the window.
The other chap watches this with amazement. After a while, his curiosity gets the better of him and he asks this guy what he's doing.
"Tigers! Keeping the sneaky devils away ha ha!", he replies.
"But there aren't any tigers in Surrey", says our man.
"Pretty effective then, eh?", replies the gent at the window...
Guy gets into a compartment on the Brighton-London train and sits down. Opposite to him is a well-dressed gent reading the FT.
Soon after they leave the station, the gent mutters something under his breath and starts tearing small squares off his newspaper and throwing them out of the window.
The other chap watches this with amazement. After a while, his curiosity gets the better of him and he asks this guy what he's doing.
"Tigers! Keeping the sneaky devils away ha ha!", he replies.
"But there aren't any tigers in Surrey", says our man.
"Pretty effective then, eh?", replies the gent at the window...
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When you pull on the yoke to increase pitch, you’re actually dumping lift*. Yes, it is at the tail which means you will also set up a rotation. Yes, this rotation will eventually mean a larger AoA and more lift. Yes, this will – eventually – reduce the rate of descent.
But right now you are dumping lift.
That is just as if you were extending spoilers. Less pronounced, but the same thing, and just when the main problem that you are trying to sort out is that you should have flared (more) a few seconds earlier to have that small extra amount of lift.
We all know what spoilers while still slightly in flight will do. No rotation required to have the RoD momentarily increase and the mains planted extra-firmly on terra firma.
Yes, all this has been said before in the thread but it seems another way of looking at the proceedings might still be in order.
Cheers,
Fred
* Or increasing negative lift, whichever makes you happy.
But right now you are dumping lift.
That is just as if you were extending spoilers. Less pronounced, but the same thing, and just when the main problem that you are trying to sort out is that you should have flared (more) a few seconds earlier to have that small extra amount of lift.
We all know what spoilers while still slightly in flight will do. No rotation required to have the RoD momentarily increase and the mains planted extra-firmly on terra firma.
Yes, all this has been said before in the thread but it seems another way of looking at the proceedings might still be in order.
Cheers,
Fred
* Or increasing negative lift, whichever makes you happy.
When you pull on the yoke to increase pitch, you’re actually dumping lift*. Yes, it is at the tail which means you will also set up a rotation. Yes, this rotation will eventually mean a larger AoA and more lift. Yes, this will – eventually – reduce the rate of descent.
But right now you are dumping lift.
That is just as if you were extending spoilers. Less pronounced, but the same thing, and just when the main problem that you are trying to sort out is that you should have flared (more) a few seconds earlier to have that small extra amount of lift.
But right now you are dumping lift.
That is just as if you were extending spoilers. Less pronounced, but the same thing, and just when the main problem that you are trying to sort out is that you should have flared (more) a few seconds earlier to have that small extra amount of lift.
Ground spoiler deployment pretty much destroys the lift over a large percentage of the wing. I would not be at all surprised to hear that it was somewhere between 25% to 50% less than a 'clean' wing. (Any experts care to comment?) That means up to half of the weight of the aircraft is now trying to accelerate it downwards.
The tailplane is a 'trimming' device. It doesn't generate anywhere near the forces the the mainplane does, nor can it. In cruising flight in a well loaded aircraft, it may not be producing any lift at all!
The aircraft (mass m) is approaching the ground at velocity v. To reduce the vertical component of v you must apply a force F over a time t. You have a choice of applying a large F over a small t or vice-versa. As we are close to the ground, t is, unfortunately, destined to remain rather small. So we need as large an F as possible in order to have an appreciable effect on v. Yes?
You are just not going to get enough force from the tailplane to have much effect on the momentum of the aircraft. If somehow you did, the primary effect would be to somersault the aircraft over it's nose or to do a complete back flip. It's about moment arms and inertia.
I think you need a lot more than a 'small amount of extra lift' to reduce a high rate of descent, and the only place that is coming from is the wings themselves with a higher AoA.
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Once I've asked in this forum why some pilots hold their nose wheel up right after landing and rolled down the runway like that for some few meters and then let the nose wheel "float" into the tarmac. Nice visual effect. But, regarding this interesting discussion, all these landings I'm referring to have one thing in common: these pilots seem to pull back the controls a little more than the required "flare attitude" fractions of a second prior to touchdown. Visually what I've seen can be represented by something like this: the plane is right on the glide path, then at about 10meters above the runway, they let the plane drop more or less 2meters twice as fast than the previous rate, and then slowly reduce this rate to close to zero, at the same time they touch the ground. Seen laterally it’s like an "S". So my uneducated guess is that the push-pull technique stated previously by Maximum is used by those pilots to get some sort of cushion that helps to smooth the landing. As pax I've felt the effects of these landings, and it’s like a small down and up rollercoaster movement, and you almost don't fell that the plane is already on the ground (except for all the panel shaking that starts immediately when rolling).
GD&L
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During a 1.2g roundout, these are reasonable figures for an actual small aircraft (9000 kg gross weight) with a forward CoG:
Mainplane lift: 110 000 N
Stab lift: -3700 N
Once floating:
Mainplane lift: 91 000 N
Stab lift: -3050 N
This is without taking into account the moment of the airfoil itself, i e only considering the pure force moments of stab lift, mainplane lift and weight.
If we assume a doubling of the stab lift, we end up with -.03g of acceleration due to the lift at the elevator.
FullWings,
it seems you're right. That's hardly enough to make matters much worse or better. One plausible explanation for why the "boeing push" feels better down - time to look into the others!
411A,
"A couple of older types, it didn't work so well....Lockheed TriStar, for example, as a few (more than a few) found out."
Ouch!
Cheers,
Fred
Mainplane lift: 110 000 N
Stab lift: -3700 N
Once floating:
Mainplane lift: 91 000 N
Stab lift: -3050 N
This is without taking into account the moment of the airfoil itself, i e only considering the pure force moments of stab lift, mainplane lift and weight.
If we assume a doubling of the stab lift, we end up with -.03g of acceleration due to the lift at the elevator.
FullWings,
it seems you're right. That's hardly enough to make matters much worse or better. One plausible explanation for why the "boeing push" feels better down - time to look into the others!
411A,
"A couple of older types, it didn't work so well....Lockheed TriStar, for example, as a few (more than a few) found out."
Ouch!
Cheers,
Fred
Experimental evidence showed that during manual landings pilots attempted to match a vertical speed / altitude profile given by a control law of the form:
Ht above runway = 3.3 x vertical speed – 12 ft.
There is a range of values for both the time constant (3.3) and the ht offset (12) that will still give a touchdown of 2 ft/sec.
The ref document RAE / BLEU ‘landings in fog’ showed two changes of landing profile with reduced visibility. First, an over-flare, which resulted in a ‘high’ float from which the aircraft quickly dropped onto the runway as airspeed reduced; some pilots tended to push over in theses conditions.
Secondly, a late flare by misjudging the flare ht. In this case:-
Either the flare was not corrected, resulting in a firm touchdown,
or with over rotation at an airspeed above the ideal, resulted in the float condition above.
or with a more extreme deviation (high VS at low alt) the crew over-rotated in an attempt to regain the flare profile (aiming for a 2ft/sec landing).
The latter condition increased the risk of a tail strike if the airspeed was lower than the ideal.
Therefore, the airspeed in addition to the visual cues available during flare (pilot perception) plays a major part in a landing. Poor judgement of height – late flare – over rotation may result a tail strike if the airspeed is low, but with ideal airspeed you may only get a firm touchdown. Thus airspeed appears to be a major parameter, especially the V^2 term for lift, and providing thrust can be applied quickly it may be the dominant effect in preventing a tail strike.
Also see the latest FSF Digest Stabilized Approach and Flare Are Keys to Avoiding Hard Landings The document requires Acrobat reader Ver 6 (available from Adobe download).
Ht above runway = 3.3 x vertical speed – 12 ft.
There is a range of values for both the time constant (3.3) and the ht offset (12) that will still give a touchdown of 2 ft/sec.
The ref document RAE / BLEU ‘landings in fog’ showed two changes of landing profile with reduced visibility. First, an over-flare, which resulted in a ‘high’ float from which the aircraft quickly dropped onto the runway as airspeed reduced; some pilots tended to push over in theses conditions.
Secondly, a late flare by misjudging the flare ht. In this case:-
Either the flare was not corrected, resulting in a firm touchdown,
or with over rotation at an airspeed above the ideal, resulted in the float condition above.
or with a more extreme deviation (high VS at low alt) the crew over-rotated in an attempt to regain the flare profile (aiming for a 2ft/sec landing).
The latter condition increased the risk of a tail strike if the airspeed was lower than the ideal.
Therefore, the airspeed in addition to the visual cues available during flare (pilot perception) plays a major part in a landing. Poor judgement of height – late flare – over rotation may result a tail strike if the airspeed is low, but with ideal airspeed you may only get a firm touchdown. Thus airspeed appears to be a major parameter, especially the V^2 term for lift, and providing thrust can be applied quickly it may be the dominant effect in preventing a tail strike.
Also see the latest FSF Digest Stabilized Approach and Flare Are Keys to Avoiding Hard Landings The document requires Acrobat reader Ver 6 (available from Adobe download).
Psychophysiological entity
Interesting. A thought experiment. A suitable mass...a golf ball sized hunk of lead, is suspended between two vertical coil springs in full sight of the pilots.
PNF. ‘Rate!' Or whatever yer SOPs tell you to scream when a couple of seconds from impact.
PF pulls back on the stick.
Providing the aircraft is still flying, the sprung mass should move downwards relative to the airframe. This means that the plane has accelerated vertically upwards relative to its own frame. (pause for groan)
The fact is, that shoving forward at the right moment, makes getting a smoothie landing a pushover on some types. (I'm sorry, it's not going to get any better. )
It's been said earlier. At that moment, the wing loading is reduced...that moment being the exact time that the nifty pilot is compressing a lot of air between him and the concrete.
A couple of other thoughts.
My recollection of Captain Highrate doing consistently good landings was cos he was going at some speed just shy of cruise. Stopping was the thing that sent the trolley with the miniatures on it up to the flight deck.
Being in a lift with a broken string, gave Einstein his ‘happiest thought', and changed the way we think about motion for ever. But when will it stop hard landings?
PNF. ‘Rate!' Or whatever yer SOPs tell you to scream when a couple of seconds from impact.
PF pulls back on the stick.
Providing the aircraft is still flying, the sprung mass should move downwards relative to the airframe. This means that the plane has accelerated vertically upwards relative to its own frame. (pause for groan)
The fact is, that shoving forward at the right moment, makes getting a smoothie landing a pushover on some types. (I'm sorry, it's not going to get any better. )
It's been said earlier. At that moment, the wing loading is reduced...that moment being the exact time that the nifty pilot is compressing a lot of air between him and the concrete.
A couple of other thoughts.
My recollection of Captain Highrate doing consistently good landings was cos he was going at some speed just shy of cruise. Stopping was the thing that sent the trolley with the miniatures on it up to the flight deck.
Being in a lift with a broken string, gave Einstein his ‘happiest thought', and changed the way we think about motion for ever. But when will it stop hard landings?
OK, time for a bit of a 'washup':
There seems to be an opinion that on some specific types a small 'push', just before touchdown, can help turn a good landing into a 'greaser'. No-one yet has come forward with a cogent explanation as to why this might be but I am prepared to take it at face value and not think of it as another 'urban myth'.
This 'breakaway' thread started when I described a heavy landing in which I was pretty much a passenger (for CRM reasons). At the last minute, well, second I started to go for the control column but we had impacted before I got there.
There were some replies saying that pulling back near the ground would make things worse and that pushing forward led to a better touchdown sometimes. I think the thread has embraced both scenarios (very high and very low ROD) and treated them in the same manner - which would explain some of the differences of opinion.
My conjecture is this:
>>>
If you are heading towards terra firma at a rate likely to cause damage to the airframe and/or occupants, the ONLY thing that is likely to rapidly improve matters is to pull back on the controls to reduce the descent speed.
If you have got so slow that this technique is ineffective then you really are a passenger.
<<<
We got sort of sidetracked into a discussion about using thrust to reduce the high rate of descent. I argued that the actual thrust itself had a much smaller influence on the outcome than might be expected, most of the positive influences were secondary ones and you had to wait some time for the effects. This is not to say that adding thrust isn't a good idea, it's just not much of a solution on it's own at a late stage.
As has been pointed out by some respondents, you can recover from a high rate of descent near the ground if you have enough airspeed to give you good control authority and a decent stall margin. If you have allowed the speed to decay unnoticed then you have effectively 'painted yourself into a corner' when it comes to dealing with an excessive sink rate.
(I remember being at Farnborough one year when a ?Dash-8? was demonstrating 'short field landings'. He came in slowly at a high rate of descent and basically crashed on the runway. Video footage showed full up elevator for the last couple of hundred feet but he was semi-stalled, so no effect... Impressively short landing, though.)
P.S. I'm off to darkest Africa this afternoon (Nigeria
) so will be incommunicado for a few days. Please talk amongst yourselves while I'm away. 
There seems to be an opinion that on some specific types a small 'push', just before touchdown, can help turn a good landing into a 'greaser'. No-one yet has come forward with a cogent explanation as to why this might be but I am prepared to take it at face value and not think of it as another 'urban myth'.
This 'breakaway' thread started when I described a heavy landing in which I was pretty much a passenger (for CRM reasons). At the last minute, well, second I started to go for the control column but we had impacted before I got there.
There were some replies saying that pulling back near the ground would make things worse and that pushing forward led to a better touchdown sometimes. I think the thread has embraced both scenarios (very high and very low ROD) and treated them in the same manner - which would explain some of the differences of opinion.
My conjecture is this:
>>>
If you are heading towards terra firma at a rate likely to cause damage to the airframe and/or occupants, the ONLY thing that is likely to rapidly improve matters is to pull back on the controls to reduce the descent speed.
If you have got so slow that this technique is ineffective then you really are a passenger.
<<<
We got sort of sidetracked into a discussion about using thrust to reduce the high rate of descent. I argued that the actual thrust itself had a much smaller influence on the outcome than might be expected, most of the positive influences were secondary ones and you had to wait some time for the effects. This is not to say that adding thrust isn't a good idea, it's just not much of a solution on it's own at a late stage.
As has been pointed out by some respondents, you can recover from a high rate of descent near the ground if you have enough airspeed to give you good control authority and a decent stall margin. If you have allowed the speed to decay unnoticed then you have effectively 'painted yourself into a corner' when it comes to dealing with an excessive sink rate.
(I remember being at Farnborough one year when a ?Dash-8? was demonstrating 'short field landings'. He came in slowly at a high rate of descent and basically crashed on the runway. Video footage showed full up elevator for the last couple of hundred feet but he was semi-stalled, so no effect... Impressively short landing, though.
)P.S. I'm off to darkest Africa this afternoon (Nigeria
) so will be incommunicado for a few days. Please talk amongst yourselves while I'm away.
Last edited by FullWings; 28th Sep 2004 at 13:22.
