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Heavy Landing

Old 18th May 2006, 20:29
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Heavy Landing

I was watching the planes landing & taking off at MAN today and was quite surprised to see one particularly heavy landing. It seemed to just drop the final 10-20 feet or so and then bounced probably the same distance back up again. The pilot then accelerated back up in the air and went around. I was amazed how high it seemed to bounce and wondered if this is common and that the undercarriage can easily cope with it? Straight afterwards a landrover began a runway inspection although this may have been a coincidence. Some inclement weather had just moved into the area at the time.
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Old 18th May 2006, 23:24
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A go-around at very low altitude (a.k.a. wave-off or aborted landing) usually results in momentary touchdown; nothing strange about that. And engines need time to spool up; nothing strange about that either.

I guess you didn't actually witness a hard bounce, followed by a go-around, but rather a bounce during a go-around - quite a different sequence of events (and certainly not as dramatic). Remember also that the sound of the engines spooling up will only reach your ears a couple of seconds after the actual event.
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Old 19th May 2006, 08:57
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But surely a bounce during a go around would have been a softer "landing" than that which seemed to be witnessed? (due to the additional power already being applied)

Sounds like a hard bounce and go-around (probably due to last minute wind-shift?) Is this common in periods of inclement weather? Must say I have never seen one.
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Old 19th May 2006, 09:54
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If you were to bounce and go 5 to 10' back into the air, then apply TOGA for a go around I would suspect that you would hit he runway again. It can take a good few seconds for the engines to spool up, I can assure you that those foew seconds can seem like an eternity as well!!!
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Old 19th May 2006, 10:37
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What is the average time of the aircraft engines spooling up from landing seting to 100% for a go around. I recall an engineer telling me that the 707 used to take 13 seconds to spool up to 100% from idle. Does this sound correct?
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Old 19th May 2006, 12:11
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Originally Posted by Doors to Automatic
But surely a bounce during a go around would have been a softer "landing" than that which seemed to be witnessed? (due to the additional power already being applied)

Sounds like a hard bounce and go-around (probably due to last minute wind-shift?) Is this common in periods of inclement weather? Must say I have never seen one.
It takes several seconds for the engines to spool up, so it is quite possible, even likely, that full go-around power was not yet available at the moment of touchdown (13 seconds for a 707 are probably not that far off, although I'd 'guesstimate' something like "twen-ty-one, twen-ty-two, twen-ty-three, twen-ty-four, twen-ty-five" myself - that surely can seem an eternity). Also, it's physically impossible to stop the descent instantaneously: that whole mass of the airplane needs to be turned around from its descent into a lift, and that takes another few seconds. This is why low-altitude wave-offs, initiated just before or during the flare manoeuvre, will result in momentary touchdown.

Does this happen often? Well, no, but it's not that unlikely either. Sudden windshifts at low altitude can occur, and I've actually witnessed a 737 doing a go-around at cat IIIa minimums in low visibility conditions, just gently touching the runway. In this particular case I still think it's more likely that the bounce occured during, not before, the go-around manoeuvre. That doesn't rule out the possibility that this go-around was initiated after the touchdown, but of these two options, the latter is just less probable.

Last edited by xetroV; 19th May 2006 at 12:32.
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Old 19th May 2006, 12:27
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Total loss at Toronto of Air Canada DC8-63, CF-TIW on Jul 5 1970 was caused by aircraft dropping after accidental deployment of spoilers by First Officer at 50ft. The Captain reacted immediately (within 1/2 sec) by retracting spoilers and opening throttles but owing to 6/7 sec spoolup time, the aircraft hit the runway very hard, causing outside starboard pod to be torn off, causing fire and loss of control.
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Old 19th May 2006, 13:59
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Originally Posted by Gordon Fraser
Total loss at Toronto of Air Canada DC8-63, CF-TIW on Jul 5 1970 was caused by aircraft dropping after accidental deployment of spoilers by First Officer at 50ft. The Captain reacted immediately (within 1/2 sec) by retracting spoilers and opening throttles but owing to 6/7 sec spoolup time, the aircraft hit the runway very hard, causing outside starboard pod to be torn off, causing fire and loss of control.
I read somewhere that this terrible accident was made all the worse by the fact that had the inadvertant spoiler deployment been made at any other time than when it was the crash would have been averted. A second or two sooner and the go-around would have been completed successfully; a second or two later and the aircraft would probably landed hard but stayed on the ground.
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Old 19th May 2006, 14:44
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Angel

HZ123,

On the old low-bypass engines there was a real danger of a rich blowout at acceleration. Meaning too much fuel could cause a flame-out. So acceleration was slow, probably around the numbers you quote.

However, on newer engines, both low- and high-bypass, that is much less of a problem, so you can just about cut the 13 seconds in half or more.

I have done quite a few acceleration checks, both after engines changes and on test flights, but I can't for the life of me remember any numbers at the moment. But 4 to 5 seconds seems to ring a bell.
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Old 19th May 2006, 21:34
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Thanks for the replies. The Airbus did drop suddenly. As it approached it seemed to flare as normal and then suddenly fell. I actually winced as I saw it drop because I knew it was going to be a hard landing. It then literally bounced back up in the air and seemed to "float" for a few seconds and then then power was applied. It remained at a similar level for a few seconds then slowly began to climb then the gear went up after that. On final approach it had already seemed somewhat wayward dropping surprisingly sharply a couple of times before picking up again. It was distincly unusual compared to the other landings I saw during the hour or so that I was watching. For the record it was a Thomas Cook flight. The reaction of people around me at the time was identical to mine with lots of expletives.
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Old 20th May 2006, 08:28
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Quite a few (all?) modern Thrust Management Systems now include a slight increase in idle power with some / landing flap extended to "prime" the engines for this eventuality by reducing spool up time.

If inclement weather was around, it was pretty gusty at MAN on Wednesday, then it is possible that there was windsheer. This is a sudden change in wind speed and or direction. The result is often a sudden drop or rise in airspeed which in the latter stages of an approach may cause a sudden increase or decrease in the rate of descent. This is particularly the case at MAN if the wind is either Northerly ish or slightly less so Southerly ish on runways 24 as the airflow is disrupted by the buildings to the north and trees and banking (probably where you were standing) to the south at low level. Yes, I know I have covered every angle but that is what makes windsheer unpleasant!

If we assume it was negative sheer, a loss in speed and an increase in RoD, then the response is a Go Around. Certainly the 757, 767 have a windsheer detection and indication which mandates a go around. I can't speak for Airbus. So, high RoD, time required to spool up, I guess I have given a long winded agreemet to the touch down in the Go Around theory as being the more likely scenario.

Good to see a sensible and genuine question rather than excitable speculation from the spotting fraternity.
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Old 20th May 2006, 09:48
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The A320 engines take about 4 seconds to spool up. If you remember the Basle-Mulhouse accident many years ago I counted 3 and a half seconds on the video before the A320 hit the trees.

Unlike a Boeing 737 the Airbus is flown manually with the autothrust engaged. You don't get porpoising, as you do with the 737, because the 'normal law' of the flight controls keeps you pointing down the approach at 1G. Therefore if you get windshear just before landing this can reduce the thrust and it can take a few seconds for it to come back. I have sat next to a guy who was convinced that the thrust was not coming back, pushed the thrust levers forward out of the CLB (climb) gate - and the thrust immediately shot up to the thrust lever angle thrust. He initiated a go around. We didn't touch down but went around for a normal landing. That was in an A330 which has a lot of thrust available, so we shot up - much to ATC's surprise.
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Old 20th May 2006, 19:22
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I had much the same experience in a 737-300. We went up like a rocket! The go-around was initiated from around 30-40 ft agl and the (rather dubious) explanation given that the cabin wasn't secure for landing!

As an aside I would love to experience a go-around in an emptyish 777 - that must be something!
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Old 20th May 2006, 20:54
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[QUOTE=Doors to Automatic]The go-around was initiated from around 30-40 ft agl and the (rather dubious) explanation given that the cabin wasn't secure for landing!
[/QUOTE

Rather dubious? If the cabin is not secure for landing the captain is obliged to go around.
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Old 20th May 2006, 21:33
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Angel CAPTAIN 'GOING AROUND'

wHERE TO?They are always going around! Usually 'the bend'!
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Old 21st May 2006, 00:47
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[QUOTE=Flap 5]
Originally Posted by Doors to Automatic
The go-around was initiated from around 30-40 ft agl and the (rather dubious) explanation given that the cabin wasn't secure for landing!
[/QUOTE
Rather dubious? If the cabin is not secure for landing the captain is obliged to go around.
I don't dispute that - but from 30-40ft???
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Old 21st May 2006, 06:35
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30 - 40ft is certainly a bit late to find out the cabin is not secure! A thousand feet is more like it. But it could happen.
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Old 21st May 2006, 11:11
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Originally Posted by Flap 5
30 - 40ft is certainly a bit late to find out the cabin is not secure! A thousand feet is more like it. But it could happen.

Guess an idiot pax could cause a cabin to be unsecured at such a late stage.
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Old 21st May 2006, 15:00
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hard landings

I read somewhere that planes are designed to withstand a "dead drop" from 6 feet or a 600 fpm impact with the runway.

It is entirely possible that you witnessed a bad landing...they do happen! even in big jets.

I recall having a new copilot making the landing in a 737. I asked him what his previous flying job was. He told me that his previous job was as an f/o on a DC8. I watched closely, though DC8 experience is a good thing.


He didn't start a round out (flare) at the normal position...nothing...nothing... nothing, then he pulled the yoke back about 8 inches! rapidly and with great force. We simultaneously hit the ground and "bounced" back into the air!

I grabbed the yoke, added some power and made the second landing on that approach.

we got to the gate and I asked him what he thought of his landing. he went into the standard bit about wanting to "learn" and for me to help him. I told him he needed more IOE (initial opeating experince) training.

so, you may have seen a bad landing. going around is also a fine solution to a bad landing in many cases.

I also watched an MD80 (from the vantage point of another plane) porpoise down the runway. new type of plane, new copilot, unwillingness to "go around".

If a hard landing is done, a write up and mx inspection is usually required. One can recognize a hard landing by the deployment of all oxygen masks, or having the flight attendent come to the cockpit with her panty hose down around her ankles (kidding !)


I also know of one case in which the loading was not accuratley computed and the plane landed at 20 knots below the correct Vref speed. The landing gear punched a hole in the wing.


Things happen. Don't let them happen to you!

jon
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Old 21st May 2006, 15:03
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Was box office witness to A340 bounce today. After said gravitational challenge event, it pitched down violently to the point it was going to become a wheelbarrow , whereupon it pitched up again and landed. I wonder how much pitch action was pilot induced, how much 'what's it doing now?'.
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