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BOAC
23rd Sep 2004, 11:50
Slightly off topic but I should add a caveat to FullWing's post
I made a grab for the control column but was too late - in case anyone should get the wrong idea, it is not a good idea to attempt to correct a potential heavy landing by pulling back on the c/c, as the wheels will merely impact even harder into 'terra firma', raising the 'g' reading. The application of a small amount of thrust will normally help to cushion such, and for the more accomplished, easing FORWARD on the stick can help (but BE SURE or don't try that!:eek: )

FullWings
23rd Sep 2004, 20:36
BOAC,

I think the subject of intervention on the controls is somewhat contentious and depends entirely on the individual circumstances.

I think that if you are heading towards the ground at 800fpm and are more than a second or two from 'touching' down, then any input to increase the AoA of the wings and start to reduce the vertical speed component is a good thing.

I also propose that the the old 'drive the wheels into the ground' chestnut is mostly a 'red herring'. (Excuse my mixed metaphors!)

The wheels are not that far from the C.G., about which you are attempting to rotate the aircraft. Look at how misloading can tip an aircraft onto it's tail for proof of that. If you work out the relative motion of the wheel units for a given change in body angle, it's pretty small.

At the end of the day it's about how the mass of the aircraft is decelerated as the suspension compresses beneath it - there is a fixed amount of oleo travel and if you use all this up your landing will become very much harder.

If the travel is, say, 5ft then you wish the rate of descent of everything else attached to the upper end to reduce to zero at 4'11 3/4" travel. Any reduction in V/S prior to touchdown will help this be achieved.

Extra thrust? Nice if you can get it but with today's big fans you'll probably get a good whoosh of power after impact! Also, if you resolve the forces, given normal touchdown attitudes, the amount of force applied to the mass of the aircraft normal to the runway surface is totally inadequate to produce any significant change in the rate of descent. I think this other 'old chestnut' comes from the days of propellors where the wash over the wing produced an increase of lift as well as thrust.

To get a jet aircraft to radically change the direction it's going in a short space of time in you have to change the AoA of the wing (thrust vectoring excepted).

Maximum
23rd Sep 2004, 21:15
I know we're off topic, but I just have to respond to your comments FullWings.

I know from experience that I can cushion an impending hard touchdown at the last moment only by an application of thrust - if I pull back on the controls, the sink rate stays the same and the landing ends up harder.

Also, quite definitely, in experienced hands a 'greaser' can be achieved time and time again: 1.) by having the correct r.o.d. in the last hundred feet or so and 2.) after the flare and just before the mainwheels touch, pushing forward slightly. (Non standard I know).

My own experience of this is absolutely in line with BOAC's.

FullWings
23rd Sep 2004, 21:42
Interesting. If you pull back on the controls, the sink rate stays the same. Does this work in other phases of flight? Maybe we should revisit the actions required for a go-around or hard GPWS?

How do you know you weren't going to get a good landing anyway the time you 'pushed forward' on the controls? I think a 'greaser' has more to do with your ROD during the touchdown versus auto-speedbrake deployment but I digress...

I'm just pointing out that if you actually analyse the physics of what you think you are doing, it might be somewhat different to the common perception...

Try this while you are on approach:

Note the rate of descent. Add thrust (keeping the attitude the same). Note rate of descent a few seconds later.

And this:

Note the rate of descent. Pull back on the yoke, keeping the thrust constant. Note rate of descent a few seconds later.

Notice any difference?

{Apologies for the creeping thread, all you lurkers!}

411A
23rd Sep 2004, 22:01
Maximum,

Yes, the 'Boeing push' generally works quite nicely, if done properly...from the very early models of the 707, to present day types.

A couple of older types, it didn't work so well....Lockheed TriStar, for example, as a few (more than a few) found out.:uhoh:

FullWings
24th Sep 2004, 00:55
I've just re-read this page and think that we might be talking at slight cross purposes about the whole 'pushing' thing.

I think BOAC, Maximum & 411A are describing a landing where the rate of descent has already been reduced to a low value and the aircraft is settling into the 'ground effect'. A slight forward movement could possibly do something strange with the tailplane/mainplane combination.

I'm talking about an unchecked rate-of-descent which will likely cause distress/damage to the aircraft and passengers. Pushing forward in this situation will do nothing to reduce the energy that has to be absorbed by the landing gear/airframe. For an extreme example see the thread on the Britannia crash at Girona.

I would strongly caution any pilot about using non-documented handling methods unless they understand exactly what they (and the airframe) are doing. If it was that good, Boeing (and others) would be recommending it. That's not to say it doesn't work most of the time, just think about what you would tell the investigating authorities when it goes wrong...

Maximum
24th Sep 2004, 01:19
FullWings, of course I'm totally aware of what happens if you pull back on the stick in a descent etc.

But the point is, we are talking about circumstances within, let's say for arguments sake, forty feet of the ground with a rapidly sinking aeroplane.

So the crucial factor you are leaving out is time. Time available to arrest the sink. If we pull back on the stick, given time, yes, it would have an effect. But before that happens, we've already touched down - harder than desired in our example.

I say again, from experience I know that the application of thrust has a much more immediate effect. (And yes it does pitch the nose up slightly on underwing engined types). So perhaps we're both really talking about the same thing here - an increase in AOA and a thrust increase - which would make sense. However, in piloting terms, I'm sure it's best thought of this close to the ground as an immediate increase in thrust that's needed.

As for pushing forward for a 'greaser', as 411A verifies, the 'Boeing push' is a well known phenomena - haven't you felt this yourself? And I know from analysing my own control inputs that it definitely works (and can also be disastrous in the wrong hands).

..............break break..............*grin*

Fullwings, I think we both posted at the same time there as I hadn\'t seen your last post.

Yes, I agree entirely, pushing forward will in no way arrest or save an unacceptably high rate of descent.

Extra thrust is what you need!;) :} :D

FullWings
24th Sep 2004, 07:47
Hi Maximum,

Yes, I think we agree on most of this.

One small point, which I think you are starting to realise:

I say again, from experience I know that the application of thrust has a much more immediate effect. (And yes it does pitch the nose up slightly on underwing engined types). So perhaps we're both really talking about the same thing here - an increase in AOA and a thrust increase - which would make sense. However, in piloting terms, I'm sure it's best thought of this close to the ground as an immediate increase in thrust that's needed.


If you read what you have written it actually says that it's the change in AoA caused by underslung engines: a secondary effect if you like. You have pitched up without elevator input. If you fly an aircraft without underslung engines, this technique has little or no effect. Also true of FBW types which take out the thrust/pitch couple (Airbus & B777 - what I fly now, so no I don't feel these effects ;)).

I still put forward that the only way to reduce the rate of descent significantly near the ground in a jet is to increase the AoA (somehow).

You will eventually get more lift from the wing from the increasing IAS, as the airframe accelerates forwards but this will take quite some time to happen. Much longer than the effects of pulling back on the stick.

I'm not saying that adding thrust is a bad thing but it isn't what is providing most of the reduction in descent rate. It just appears that way. :D

It's basic Newtonian mechanics. You have a large mass of aeroplane moving towards the ground in the vertical plane. To reduce this rate of descent you must apply a similarly large force over a period of time in the opposite direction. Think about where this comes from...

Safe flying all :cool:

BOAC
24th Sep 2004, 08:34
All the above points are vaild, but a late 'rotation' or 'grab' WILL increase the INSTANTANEOUS 'g' at main-wheel touch, and could well put the QAR reading into the 'coffee-no-biscuits' area. Of course a pitch change at the RIGHT time is better, but we are talking about when it is 'too late'. Bringing Gerona into this is a REAL red herring. I can assure you, that having rescued a few co-pilots' potential 'heavies', 'grabbing' late is NOT a good option. All I am saying is a warning that a late intervention can often make things worse.

I 'co'd' for a very experienced Captain a few years ago who ALWAYS had the most 'horrendous' descent rate all the way to landing - such that I had the leather gum protector in my mouth - and he ALWAYS greased it on:confused: I STILL cannot work out how he did it.

FullWings
24th Sep 2004, 21:27
I'll have to get my apologies in first for hijacking this thread - it's a shame we can't branch off this section into 'Tech Log'.

I find this sort of discussion fascinating because normally pilots are quite technical in their outlook, i.e. knowing the Flying Manual, checking the weight and balance to 1kg, setting Vref to within 1kt, etc. Then you start discussing landing technique and you end up with a sort of PMT: "It just does, OK!!!";)

Being of an inquisitive mind, I like to understand the reasoning/logic/physics behind what we do. Not that it seems to make me any better at it but I like to anyway.

Here's a multiple choice question (tech quiz, say):

You are approaching the ground at a higher-than-optimum rate-of-descent. Do you:

a) Pull back.
b) Push forward.
c) Sit on your hands.

I hear what several of you are saying: If you try and save a bad landing by pulling back at the last minute, it makes it worse.

This is interesting from a scientific point of view because you have only done one landing, whereas to make a comparison you would have to have done two from exactly the same starting point. I suppose I am trying to say, HOW do you know it made the landing better/worse if there is nothing to COMPARE it with? WHY should it make it worse? Let's have some sines and cosines and equations of motion...

We all know every landing is different, so it doesn't wash to say "But we did XXX at YYY and thumped it in so..."

There are a lot of 'old wives tales' floating around in aviation and it's very easy to start believing in them. Especially if your Flying Instructor (God + 1) told them to you.

People will generally believe what you tell them. Especially if things seem to work in a way consistent (on the surface) with the explanation. I reckon you could teach someone that the Rudder was the primary means of turning the aircraft and the ailerons were there to stop you wobbling all over the sky as you turned. The theory is mostly consistent with the practice but we all know it's not quite correct.

Maybe one of us should try 10 landings in the sim from a frozen position and see what happens with different control applications. This might produce some results but again, sim manufacturers are notorious for slack programming at the edges of the flight envelope.

Maximum
25th Sep 2004, 11:00
Terrible case of thread hijack this, but anyway.:\

What about 4.) Add thrust.

Just very quickly, without thinking about it to much:}, I think part of the reasoning is in the fact we have a high sink rate - perhaps because we've let the speed wash off to below Vref. We're now well on the back of the drag curve, so pulling back simply increases the drag and we slam it in.

Also, I know from experience that if you pull back at just the wrong moment you do rotate the gear harder onto the runway. It may be a small moment, but it certainly has an effect.

And thirdly, as I mentioned before, there just isn't enough time with a high sink rate and a late pull for the aircraft to change it's vector into the tarmac.

Proof of this is also seen in the case where you have a high sink rate but a margin of speed over Vref. In this case the landing can be saved with a pull (as long as it's not too late) as the lift increases rapidly with increase in AOA due to the excess speed. The ground cushion is also more effective in this case - think of the float one gets with a flapless landing.

All the above applies to certain jet transport types, not to light aircraft with so much less inertia.

Sorry there's no equations - not my strong point! :O

FullWings
25th Sep 2004, 12:08
Add thrust. Hmmm.

Firstly, I would like to state that I too think it would be beneficial but the question is by how much? Well, better to give it a 'handful' than do nothing...

If you get so slow that increasing the AoA on the wing doesn't give you any more lift, there is a word to describe the situation. Stalled! So at that point, you are doing a 'stall recovery' - full power, etc. When I fly something like a Piper Cub, I try and stall it a few inches off the ground. I definitely do NOT do this in a jet transport. Not only would you run out of runway but the tail would probably hit the ground.

I'm thinking of force resolution. You need somehow to reduce the vertical component of the aircraft's motion so it doesn't hit the ground so hard.

If we examine the act of adding more thrust more closely it gets interesting. Take the B777 (just because I have some of the data next to me). Go into the 'flight with unreliable airspeed' section and look up the landing config. body angle at a normal landing weight: -0.3 degrees, i.e. slightly nose down. This is almost identical to the angle when sat on the ground, so the extra thrust has a direct effect on the vertical motion of the airframe of approximately...nothing! I would assume that other jets are not that far away from this in terms of body angle.

So, we are now looking at secondary effects. Underslung engines will give an 'uncommanded' pitch-up moment with increasing power. (Not on the 777 as the FBW deliberately counteracts this.)

We are left with trying to increase the airspeed to get more lift out of the wings. Not only does this take time (you are in a high-drag configuration) but the percentage increase in lift for a 5-10kt change in IAS is quite small because of the higher approach speeds of jet aircraft.

{Explanation: I am coming in to land at 40kts in my 'cub. I apply some more power to pick the speed up to 45kts. The lift from the wing (all else being equal) has increased by (45/40)^2 = 26%. I now do an approach in the 777 at 140Kts. An increase of 5kts IAS will give me (145/140)^2 = 3.5% more lift. Not a lot, really and certainly nowhere near enough to affect the outcome.}

Where does that lead us? I still remain skeptical about anything other than pulling back on the controls but remain to be convinced...

BOAC
25th Sep 2004, 13:09
I´m asking the mod for this forum to split this when time available as it is definitely going ´thread creepy´:D

landing config. body angle at a normal landing weight: -0.3 degrees, i.e. slightly nose down. FW - are we getting attitudes confused here - is that the ´landing attitude´or the ´approach attitude´? If the former, I would expect loads of nosewheel first touchdowns? I would venture to suggest that most a/c land in a nose high attitude - also what is the engine mounting angle?

My MAIN point is to try to discourage last minute ´grabs´such as you describe. Having sat through endless ´circuits and BUMPS´in aircraft from 152 through Jet Provost to just ´BUMPS´ in Lightnings and 737s, I can assure you that such ´grabs´are not helpful in all but the first two types. That is most definitely NOT an OWT.

If you have any doubts about the ´physics´ of rotational kinetic energy, take one of your most expensive and fragile wedding dinner plates, rest it on a suitable pivot (NB Zero descent rate), and then whack one side of it downwards onto a concrete floor.

To guarantee this thread going to JetBlast :D what about " you are in a lift which is falling down the shaft. Just before the lift impacts with the bottom of the shaft, you manage to spring upwards at the same speed as the lift is falling." Discuss:D

flapsforty
25th Sep 2004, 13:59
Always welcome BOAC! ;)
I'm sure mr Draper would have a considered answer to your hypothetical.

Flight Detent
26th Sep 2004, 12:28
Hi all,

I'm confused again, we are talking about a 'hard' landing here are we not, otherwise the best suggestion I have to fix a 'heavy' landing is to go out yonder and dump some fuel!

Cheers

PS - the best fix for landing a -200 series B727 was to push slightly just prior to touch, a little thrust always helped if the descent rate was a bit large!
As I understand it, the -200 B727 was one of the most difficult to consistently land with only the required amount of 'bump' at touchdown.

FullWings
26th Sep 2004, 12:49
Thank you Mr. Flaps, it was starting to get embarrassing.

Discussion mode re-enabled.

Part I:FW - are we getting attitudes confused here - is that the ´landing attitude´or the ´approach attitude´? If the former, I would expect loads of nosewheel first touchdowns? I would venture to suggest that most a/c land in a nose high attitude - also what is the engine mounting angle?Well, I suppose if you wanted to be really pedantic, you would say neither! As I understand it (and am ready to be corrected) you have 'Approach Configuration' which will normally involve a bit of flap but no gear and 'Landing Configuration' which involves the gear and all the flap you are planning on using. 'Landing Attitude', I would suggest, would be the final pitch achieved during the flare, just before touchdown. Subtle differences I know but you were right to point in that direction.

Looking at the manuals again, I see on the 777 that a 'Standard Landing' involves initiating the flare at c.20R and raising the nose by one to two degrees. At the moment of touchdown the angle is c. four degs. Looking at the aircraft sitting on the ground and walking round the engines, comparing the jet pipe to the ground, I can't actually see any evidence of them being mounted other than pointing straight back, although I admit I don't know how good I am at judging these sort of things.

Let's do some maths. :D

Come back everyone! :{

Now, before we begin, let's list the assumptions:

1) 'Thrust Angle Relative to Ground'. OK, four degrees touchdown attitude rounded up to 5 in total.

2) We use the most powerful variant of the 777 that I fly (Trent 895).

3) The engines spool up instantaneously to G/A thrust. (190,000lbs.)

4) Rate of descent at 25R is 700fpm - consistent with a landing weight of c.180T on a 3 deg. approach.

5) The airframe immediately assumes the touchdown pitch attitude when commencing the flare.

I think these are pretty conservative assumptions and all err on the plus side (as far as the effects of thrust on landing go...)

Resolving forces in the vertical plane...

Component of thrust (Kg) = Sin(5 degs) * 190,000 / 2.2
=> 7,500 == 7.5T

From Newton's 2nd Law, F=ma...

75 (KN) = 180,000 (Kg) * a
=> a = 0.42ms^-2 or around 1 & a third feet per second per second.

At that rate it would take a little under four seconds of FULL POWER, just to reduce the descent speed from 700fpm to 400fpm. There is an effect but it's not as great as we might have hoped. (OK, so if it was a 'normal' landing, being at the attitude described, you should have experienced a 'normal' touchdown but I'm just using these figures to show what real effect the thrust is actually having on the rate of descent.)

By raising the nose with the elevator, you can probably produce forces an order of magnitude greater in a much shorter timespan.

In real life, the engines would take some seconds to spool up to that level and the attitude would be somewhat lower to start with. Also, we are describing a twin with a pretty good power-to-weight ratio. Finally, would you be ready (mentally) to use TO/GA power at this point?

Part II to follow after the interval :D

FullWings
26th Sep 2004, 14:10
If you have any doubts about the ´physics´ of rotational kinetic energy, take one of your most expensive and fragile wedding dinner plates, rest it on a suitable pivot (NB Zero descent rate), and then whack one side of it downwards onto a concrete floor.We didn't get any plates at our wedding but the hotel did have carpet. ;)

Take a model of an aircraft and do the same to it. Oh, the tail has hit the floor.

As I ventured in a previous posting the main gear on most passenger jets is attached fairly close to the C.G. (Otherwise you'd never get the buggers off the ground.) Using 'back of the fag packet' geometry, if the main gear was 10 feet behind the CofG and you pitched up an extra five degrees (actually enough to get a tailstrike on the 777), then the wheels would move about 11 inches (and not all of that in the vertical plane). Compared with everything else that is going on, I don't think it's going to make a huge difference to the smoothness of the landing. This example is for a 200' long aircraft - on smaller things like the 737 the effect would be much less.

" you are in a lift which is falling down the shaft. Just before the lift impacts with the bottom of the shaft, you manage to spring upwards at the same speed as the lift is falling." DiscussHow could I refuse such an invitation? :D

Well, assuming you are using your legs to propel you away from the lift (accelerating it further in a downwards direction as you do so), I don't see much point in doing it. Why? Because the acceleration you are giving yourself to reduce your velocity to zero w.r.t. the lift shaft is pretty much the same as you would experience when you use those same superhero legs to take the impact of the lift crashing to a stop...

Wino
26th Sep 2004, 15:17
You are forgetting one more thing about pushing vs. Pulling.

The tail plane exerts an enourmous DOWNWARD vector of lift on the aircraft (on an A320 somewhere around 40,000 lbs of force are applied DOWN on the tail to keep the nose in a steady state) That 40,000 lbs of downward force is required to be overcome by the wings as lift in addition to the weight of the aircraft inorder to lift the whole kit and kaboodle off the ground.

When you "push over" you instantly lighten the aircraft by some percentage (rather large) of that 40,000lbs. That will have a momentary lifting effect on the aircraft before the reducing angle of attack overcomes it and the aircraft starts down hill. It is quite noticable.

There are other things guys don't notice in every day flight either. You MASH the rudder for example and the aircraft will roll the otherway first (a few degrees) before the yaw developes and the aircraft rolls in the direction of the rudder.

Cheers
Wino

safetypee
26th Sep 2004, 16:22
Wino I suspect that you and other contributors to this thread are seeking a generic answer or are being too specific about an individual aircraft type. An aircraft's response to pitch control input will vary with aircraft type (and configuration) – the short period motion. Also, so will the effect of thrust, angle, amount, engine response time, etc.

For those who have used several techniques across different types I suspect that the nose down pitchers, at best, will stop the nose up pitching motion (rotation = zero), and thus if the residual attitude is less than that for tail strike a satisfactory landing will be made. This of course is speed dependent, an attitude less than for a tail strike at low speed may still result in a strike after oleo compression if there is reduced lift due to slow speed.

For the thrusters, this can be a powerful effect, not only from vectored thrust, but also ‘recirculated’ lift for wing mounted engines, and the lift from any speed increase. The effectiveness of these varying by aircraft type.

And then there is ground effect!

The nearest generic procedure that I have seen is a combination of sharp pull, check, and immediately remove what you put in (push), with a touch of thrust. Then for a co-pilots landing, a firm hand preventing further rearward stick movement with a thrust increase.

FullWings
26th Sep 2004, 17:08
The tail plane exerts an enourmous DOWNWARD vector of lift on the aircraft (on an A320 somewhere around 40,000 lbs of force are applied DOWN on the tail to keep the nose in a steady state) That 40,000 lbs of downward force is required to be overcome by the wings as lift in addition to the weight of the aircraft inorder to lift the whole kit and kaboodle off the ground.

When you "push over" you instantly lighten the aircraft by some percentage (rather large) of that 40,000lbs. That will have a momentary lifting effect on the aircraft before the reducing angle of attack overcomes it and the aircraft starts down hill. It is quite noticable.I don't have any figures to hand but a downforce of 40,000lbs on the tail sounds rather a lot for an aircraft of only 93,000lbs empty weight? Is the aircraft so out of balance on the final approach it needs this huge corrective input?

Also, if these figures are true and you are reducing this amount by a rather large percentage the pitch-down couple is going to be huge :ouch:

Help! We need some aerodynamicists!

I like safetypee's response best:

The nearest generic procedure that I have seen is a combination of sharp pull, check, and immediately remove what you put in (push), with a touch of thrust. Then for a co-pilots landing, a firm hand preventing further rearward stick movement with a thrust increase.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?

Maximum
26th Sep 2004, 17:39
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?

No, nothing to do with tail strikes. Simply to do with the fact that a small, and I mean 'small' push a fraction of a second before the mainwheels touch will produce a 'greaser' on many occasions. To keep it specific, in this case I'm talking about the B737-300.

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!

:}

FullWings
26th Sep 2004, 18:10
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. :D

When I sit on RW05 @LHR, waiting for my turn to cross :bored: 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.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...:confused: )

Maximum
26th Sep 2004, 19:36
I used to fly the 737 and I don't remember ever doing that - maybe I should have!

with all due respect, there's our sticking point FullWings - you've never used the technique! :D 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.

...ah, but there's the rub...how many times do we do this, holding the attitude, convinced it'll kiss the tarmac, only to be surprised by a thump.

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........ :8

john_tullamarine
26th Sep 2004, 23:31
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.

Maximum
27th Sep 2004, 10:04
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.

FullWings
27th Sep 2004, 10:58
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...

ft
27th Sep 2004, 11:53
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.

FullWings
27th Sep 2004, 12:41
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.Yes, technically that is precisely what you are doing. I think the discussion here is around how much lift are you actually getting rid of, or to simplify, what is the change in the force applied?

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.

GearDown&Locked
27th Sep 2004, 12:44
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

ft
27th Sep 2004, 13:54
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

safetypee
27th Sep 2004, 19:02
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 (http://www.flightsafety.org/pubs/fsd_2004.html) The document requires Acrobat reader Ver 6 (available from Adobe download).

Loose rivets
28th Sep 2004, 07:00
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?

FullWings
28th Sep 2004, 11:34
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. :D)

P.S. I'm off to darkest Africa this afternoon (Nigeria :uhoh: ) so will be incommunicado for a few days. Please talk amongst yourselves while I'm away. ;)