Randomized touchdown points for autoland

[AnthonyGA]

Years ago, I read that ILS autoland systems were originally so precise that they would land an aircraft on exactly the same spot on a runway over and over. Supposedly this put uneven stress on runways, wearing out one spot while the rest of the runway remained in acceptable condition. And supposedly autoland systems were modified to slightly randomize the exact touchdown point for each landing so that it wouldn't always be hitting the same spot.

Unfortunately, I can't remember where I read this. It sounds a bit like an urban legend. Do autoland systems really randomize the touchdown point for each landing?

[FE Hoppy]

The term is "longitudinal scatter" and I heard this whist doing my L1011 conversion at Cranebank back in the 90's. I spoke to Honeywell/Embraer engineers about this and they laughed!

[STBYRUD]

If I built the 737NG FCCs I would also claim that I deliberately introduced error into the system to 'prevent uneven wear on the runway' Best excuse ever

[grounded27]

Despite this not being possible due to environmental factors, different aircraft types land at different runway positions in autoland due to tail clearance, flare etc.

[decurion]

If you look at many autolands you will see a clear touchdown dispersion both lateral as well as longitudinal. It will never land on the same spot every time. During certification the manufacture must show that the aircraft can land within a fixed landing box with a certain probability.

[stilton]

Cue, you know who..

[TheWanderer]

With increased GPS precision and ground based GPS position correction, one can nowadays decide to land right or left of the centerline.
To prevent holes on the runways due to all aircraft landing on the exact same spot, precision GPS signals are scrambled again and acheivable precision is reduced.

[BOAC]

anthony - while the 'aiming points' may be of a high order of repetition, I doubt very much that the variables in the flare manoeuvre would cause a precise touchdown point.

Remember, of course, the Trident nosewheel was offset to avoid damage to the centreline lights due to the high precision of its autoland system.

[411A]

Remember, of course, the Trident nosewheel was offset to avoid damage to the centreline lights due to the high precision of its autoland system.

Ahhh, gotta love this support of a Brit aeroplane, even IF the information is rubbish.
Fact is, the HS.121 nose gear retracted 'sideways' simply because...there was no where else to put it, due to equipment bays being...in the way.

However, there is no such 'offset' with present day airline aircraft, for autoland maneuvers.
However, during trials of the afore mentioned HS.121 autoland system, the glidepath signal was offset slightly, so as to avoid pavement damage, due to very precise touchdown position.

How do I know?
DP Davies mentioned same, several times....and I expect he would have known for sure.

[forget]

.... the glide-path signal was offset slightly, so as to avoid pavement damage, due to very precise touchdown position.

That makes no sense at all. If aircraft systems were sufficiently accurate to consistently hit the same spot then off-setting the Glide-Slope simply moves the spot - they'll still all hit it.

[Starbear]

Autoland Longitudinal Scatter Distance

I too recall this term during my L1011 time but I never came accross any claims it was a deliberate bias or such. Others have touched upon the real reasons.

Given that the aircraft does not follow the G/S all the way to touchdown; once it starts the flare, it will of course depart the G/S and is no longer following any vertical guidance and so be subject to, as others have indicated, many variables. It could of course then be subject to float with no auto method of correcting, except perhaps a little from the autothrottle if operating and then usually only for speed control. During a manual landing, the pilot could "cheat" a little if he gets concerned.

The landing performance tables/graphs for that aeroplane and every other machine with autoland that I have flown added an additonal distance to the field length required for an autoland. And this was because of the imprecise touchdown point but usually it addded about 1100 to 1300 ft to the calculations could be much more in some cases or a bit less. This being based upon the G/S angle and threshold crossing height of the G/S signal.

[BOAC]

Originally Posted by 411A

Fact is,

- I take it you are also totally boring to work with?

[Graybeard]

As I heard it: When BA were documenting the first 1,000 autolands with the L-1011 at LHR, the engineers noted the very small amount of scatter of the touchdown points, so they asked the airport authority for an opinion. The panicked reply caused the Collins/LearSiegler engineers to program a bit of scatter in the touchdown point.

If it were really important, I could ask an old retar'd co-worker who was one of the design engineers at the time.

GB

[forget]

Old wives tale - for half a dozen different reasons.

[Double Hydco]

When I did the type rating, I think I remember being told that when the RAF did the Belfast autoland trials (it had the same Smiths autoflight system as the Trident) the aircraft kept touching down in the same spot, and the touchdown zone at RAF Fairford had to be reinforced as a result.

DH