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Aircraft Touchdown Accuracy

07/27/2013 7:38 AM

Dear respected gurus-how accurate is a big aircraft like boeing 737 touchdown?

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#1

Re: aircraft touchdown accuracy

07/27/2013 8:07 AM

That is pretty much up to how good the pilot is.

Cross winds and up/down drafts (A.K.A. weather) is the complicating factor. Those conditions change along the flight path's final approach and can be difficult (if not impossible) to predict, so, the pilot is always compensating for those variations.

For a fully automated landing of the 737 (i.e., augmented GPS - theoretically speaking), the limit is going to be directly proportional to the accuracy (vertical and horizontal) of the navigation system guidance. Instrumented landing guidance requirements are generally accurate to less than 1 meter vertically and 4 meters horizontally (Category IIIA). Actual results are much better than that, but the determining factors are pretty complicated, so a worst case scenario is what is used for the requirement.

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#2
In reply to #1

Re: aircraft touchdown accuracy

07/27/2013 8:31 AM

Thank you.what about the latitudinal accuracy?

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#3
In reply to #2

Re: aircraft touchdown accuracy

07/27/2013 9:42 AM

Augmented GPS Horizontal accuracy is better than 4 meters, so that holds pretty well for most airports anywhere on the globe.

I suspect that as you approach the poles that you may see a degradation due to the number of available satellites or areas surrounded by mountains that may also reduce the number of satellites, but airports employ other systems to augment an approach.

However, there are other ILS-type systems that are employed at airports that can replace Differential GPS, such as MLS (microwave), TLS (transponder), LPV (localizer performance and vertical), GBAS (another ground based system) and laser. There is a new system called LAAS (local area augmentation system) in the works, but that has been developed after I left the field of aircraft precision approach in my last job, so I just don't know much about it.

Each system is different and every airport uses a different combination of systems. It depends on a number of selection factors as to what certified systems they install.

When I left the field, WAAS GPS (wide area augmentation system) was the hot ticket and still is.

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#4

Re: Aircraft Touchdown Accuracy

07/27/2013 1:49 PM

It just has to be "close enough".

This (UNRELATED) might be of interest to some. It could explain what went wrong in FLT 214, not a 737; Were Asiana Pilots Caught In The FLCH 'Trap'?

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#5

Re: Aircraft Touchdown Accuracy

07/27/2013 1:49 PM

Take a look at any large airport runway landing zone and you will quickly see your answer from the touch down zone wheel marks. These can be several hundred feet in length.

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#6
In reply to #5

Re: Aircraft Touchdown Accuracy

07/27/2013 2:40 PM

That touchdown point is really arbitrary. There is no hard fast rule where the pilot must make contact on the tarmac.

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#18
In reply to #6

Re: Aircraft Touchdown Accuracy

07/27/2013 5:58 PM

Thats not quite true, some of the large airports have specific areas for the heavies to set down at. They are marked as such. Look in the FAR/AIM manuals and it will show those details.

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#7

Re: Aircraft Touchdown Accuracy

07/27/2013 3:09 PM

737 is not a big aircraft (or is it?)

How accurate is accurate?

If the wheel bearing tolerances are in microns, and the wing dimensional tolerances in millimetres, and the runway is 40 metres wide, and 4000 metres long, what do you mean by how accurate is touchdown?

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#9
In reply to #7

Re: Aircraft Touchdown Accuracy

07/27/2013 3:19 PM

OKAY- Let us put things on the table: what is the pilot's ability to land, so that gap between the 2 wheels of the nosewheel will be above the white line?

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#11
In reply to #9

Re: Aircraft Touchdown Accuracy

07/27/2013 3:28 PM

#6

Every landing is different.

Any landing you can walk away from is a good one.

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#13
In reply to #11

Re: Aircraft Touchdown Accuracy

07/27/2013 3:34 PM

Thank you .Have a nice day.

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#22
In reply to #11

Re: Aircraft Touchdown Accuracy

07/28/2013 2:02 AM

Someone described a landing to me as a controlled crash. But he also said takeoff is more dangerous than landing. Tell you what it doesn't feel like that, with a landing plane feeling like a boat ride with a small outboard motor on choppy water.

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#23
In reply to #22

Re: Aircraft Touchdown Accuracy

07/28/2013 5:13 AM

Landing on an aircraft carrier is a controlled crash, landing on a runway is a landing.

As most of the posts have indicated, what exactly does the OP mean.. 'How accurate?".

The plane lands on a strip of tarmac that at 5000ft looks no wider than a piece of hair.. and everyone walks away. Good landing I say!

And what qualifies me to say that? I catch every 5 weeks 5 flights to get to and from work... so I know a little bit about landings.

As for takeoff's.. yes slightly more dangerous as the plane has a lot of fuel on board...

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#25
In reply to #22

Re: Aircraft Touchdown Accuracy

07/28/2013 8:41 AM

More like a controlled stall, than crash.

Every airborne aircraft is guaranteed a landing.

Landing approach begins with a nose down angle of attack until it nears the point of the runway where it performs what is called a flare and the nose is brought to a positive angle of attack and the aircraft skims above the runways until its speed drops enough to "land" as lift diminishes.

Takeoff is generally more perilous because you are relying heavily on the engines to perform at a higher level than during flight or even landing. Any loss of power is very tricky and could result in a stall due to the low initial airspeed.

Airports do not have the luxury of a second emergency runway straight ahead if something goes wrong on takeoff, so a lot of attention is payed to getting everything right.

Even if they did have that runway, if the aircraft velocity is too low it can stall. Usually one wing stalls and it causes the aircraft to roll, which accelerates the problem and also points the aircraft in a different direction.

When flying sailplanes we are taught to sense when a stall is approaching and if the aircraft does stall, the solution is to quickly drop the nose of the aircraft and apply opposite rudder. Opposite rudder means that whichever wing drops first, push the opposite rudder pedal all the way in to turn the aircraft and thus increase airflow over the stalled wing.

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#14
In reply to #9

Re: Aircraft Touchdown Accuracy

07/27/2013 3:43 PM

How accurate is the white line?

What is meant by "How accurate is the white line"?

How accurate does it have to be as long as the wheels land on the tarmac, with enough runway left to come to a halt?

Is it better to have the wings horizontal, but the nosewheel off-line, or vice-versa?

Regulators for Airports and Air-traffic use extremely generous safety margins. As such, I would say your original question is rather meaningless.

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#15
In reply to #14

Re: Aircraft Touchdown Accuracy

07/27/2013 3:52 PM

We have an idea to harvest the kinetic energy of a landing aircraft [some ideas developed by others were patented already]So it is necessary for us to know the landing accuracy, otherwise, the device which we plan will be too clumsy!

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#16
In reply to #15

Re: Aircraft Touchdown Accuracy

07/27/2013 4:20 PM

You will have to do this by your own observation then. Only you know what you mean by accuracy, or what it means to you. No airline, airport or aircraft manufacturer has ever had an issue with the "in-line" accuracy of landings (as opposed to falling short, or over-shooting) compared to runway size, so it has never been documented. For the aforementioned it has never even been worth thinking about.

Only you can see if the average wheel landing area is small enough for your requirements, and enough to make the idea cost-effective.

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#46
In reply to #7

Re: Aircraft Touchdown Accuracy

07/29/2013 12:21 PM

737 is a narrow body (not even a wide body!) so, I would say: No, it is not a "big" aircraft.

I think it is the largest size of narrow body. But there are many much larger aircraft.

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#8

Re: Aircraft Touchdown Accuracy

07/27/2013 3:15 PM

To get the REAL feeling of the difficulties of landing an aircraft, buy an hour in a simulator.

You can dial in aircraft type, wind, clouds, light and other conditions.

Just lining up with the runway on approach might take the whole hour.

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#10
In reply to #8

Re: Aircraft Touchdown Accuracy

07/27/2013 3:25 PM

Dear lyn- don't you think that for better results you should recomend me to take some kid and tell him to practice this landing on a simulator, instead of taking an 75 years old guy to practice this?!

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#12
In reply to #10

Re: Aircraft Touchdown Accuracy

07/27/2013 3:33 PM

He'll make it look easy.

You do it. That way you will appreciate what the pilot does.

I do it and I'm 66. It's cheaper than renting a plane, and you can blow all the landings without making any smoke.

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#17
In reply to #12

Re: Aircraft Touchdown Accuracy

07/27/2013 5:06 PM

Now try that on a pitching aircraft carrier.

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#19
In reply to #17

Re: Aircraft Touchdown Accuracy

07/27/2013 6:03 PM

Try that with a 2-5 hour student, thats when you really learn to fly and only sweat on the right side of your face. LOL

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#26
In reply to #12

Re: Aircraft Touchdown Accuracy

07/28/2013 9:58 AM

Well lyn- I considered the situation. Since I'm financing the music education of my daughter moran, I 've no excess money [75yrs old, retired].

So I phoned Yosi Zin who was born and raied up in my kibutz, and for 3 decades is an airliner pilot [757], after 2 minutes he gave me all the information I needed.

The reason I had not done it before was that yesterday was saturday -our shabat, a rest day even for atheists like us.

Thank you and all the other folks who tried to help me!

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#20

Re: Aircraft Touchdown Accuracy

07/27/2013 7:14 PM

Accuracy is one thing, but what is it when the ''tower'' wasn't successful in telling any of the four pilots on-board that their aircraft that recently (essentially tripped) over the sea-wall while trying to land at San Francisco International was on an approach flight-path that was too low while they still had room to correct said approach?...

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#43
In reply to #20

Re: Aircraft Touchdown Accuracy

07/29/2013 4:19 AM

It would have been interesting to try harvesting the kinetic energy out of that one.

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#52
In reply to #43

Re: Aircraft Touchdown Accuracy

08/10/2013 7:29 PM

How ''interesting'' would it have been to you if all the passengers and crew had been English?...

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#21

Re: Aircraft Touchdown Accuracy

07/28/2013 1:29 AM

From the NTSB concerning the recent 737 which had the nose gear fail on landing. Plane was landing with a -2o horizontal angle (nose lower) while it should have been at least +3o (nose higher). This means on a tricycle landing gear (such as the 737 has but only bigger) you should make initial contact with the rear landing gear first and then the nose gear as your speed reduces. Sample of what one of the accuracies that are required and the effect of not complying with it.

Good Luck, Old Salt

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#24

Re: Aircraft Touchdown Accuracy

07/28/2013 7:19 AM
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#27

Re: Aircraft Touchdown Accuracy

07/28/2013 10:10 AM

I thought it was interesting comment by one of the people involved with the recent crash at San Francisco Airport that they had just moved the touchdown line 300 feet inland from the water. Had they not done that, the plane may have hit the wall at the waters edge and it would/could have been much worse. Moving that line was also why the landing guidance was not up and running completely yet.

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#28

Re: Aircraft Touchdown Accuracy

07/28/2013 3:40 PM

there must be a flight academy in israel - why dont you say Hello to them

i donno - just checked for personal interest how far is that information

old news

perhaps they know who you should ask

2008 - related terms

e.c.

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#29
In reply to #28

Re: Aircraft Touchdown Accuracy

07/28/2013 3:56 PM

read my 26# answer to lyn!

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#30
In reply to #29

Re: Aircraft Touchdown Accuracy

07/28/2013 6:00 PM

I'll do a loop for you next time I go into combat.

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#31

Re: Aircraft Touchdown Accuracy

07/28/2013 6:08 PM

Mr. AZ-

My first posted response somehow disappeared - went into oblivion! So I'll to try to explain here it again!

Long before the GPS technology came about, there are various navigational aid system that are already put in place to assist pilots/navigators. These are use for orientation, or for safely landing their aircraft back to earth. Systems such as Instrument landing systems (ILS), VORs, Glide Slopes, etc. have been in place then and up to now I assume are still being use in conjunction with the modern GPS technology.

Most of the airports I believe are equipped with the basic ILS instruments to help guide the pilots safely bring in or land an airplane even during bad or inclement weather conditions! The ILS are designed to continuously communicate with the pilots through a transponder on every plane. An ILS transmitter works in such a way that it produces, broadcasting a directionally beamed signals from the ground at the end of the runway. It will look like an ice cream cone where the pointed tip is located at the end of the runway!

These broadcasted signals will appear, shaped like a hollow ice cream cone, divided into 4 quadrants by a series of tones or signals forming an "X and Y" planes which appear like a cross at the center of the "hollow cone shaped" signal envelope. This signal beamed upward is also gradually sloped at a predetermined approach angle, serving as a glide path during landing. Inside this "beamed tunnel" signal are other signals arranged in such a way like in a cross patterned "X and Y' coordinates. That any deviation from its center, either left or right, up or down, the pilot/navigator will hear or receives a varying toned signals depending on where the plane's nose is heading and deviating. The pilot's objective is to aim the airplanes nose' dead center and to coincide with the center or the zero crossing point of the "XY" axis. This point is where the signal will be nulled! This quiet zone "where signals cancels each other" is the nulled point" that pilots needs to follow to safely land the aircraft. On much modern aircraft that are on autopilot mode can remain on autopilot even when landing using the GPS technology in combination with the older technologies!

In my opinion, the aircraft landing accuracy when using the above-mentioned systems will only be dependent on the pilots ability to react and follow the transmitted signals!

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#32
In reply to #31

Re: Aircraft Touchdown Accuracy

07/28/2013 6:22 PM

You said, "systems will only be dependent on the pilots ability to react and follow the transmitted signals!"

And the pilot's ability to be aware of the surroundings, heading, airspeed and altitude of the aircraft, on approach.

What your they doing?

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#33
In reply to #32

Re: Aircraft Touchdown Accuracy

07/28/2013 7:34 PM

My understanding was, as they come near their intended destination, they will be in contact with the area air traffic controllers who's duty is to give them instructions, directions and other pertinent air traffic conditions. Beside their visuals, they are also monitoring their instrumentation measurements, validating and comparing visual observations with data received from transponders and other electronic instrumentation. If they are on autopilot mode, air speed, altitude, directions are automatically fed and guided by those navigational aid systems. Otherwise during manual mode, those parameters you cited has to be entered manually and continuously adjusted by them, as instructed and coordinated with the local air traffic controllers.

Once they're in the immediate destination vicinity, and the air traffic controllers had given them the green light, they can basically bring their aircraft positioned right at the very mouth or opening of that coned signal and follow it downward! If the ILS is available, they may use the system's information as a tool to guide them beside their visuals.

On manual mode, the accuracy is on how fast they can react and how steady they can bring and maintain the aircraft's nose to follow the 0 axis along the glide slope until they hit the runway.

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#34
In reply to #33

Re: Aircraft Touchdown Accuracy

07/28/2013 7:54 PM

Your understanding is incorrect, and lethal. It is the pilot's responsibility to be aware of conditions at all times.

There were three senior pilots in the cockpit when the aircraft was flown into the ground.

If they were, "monitoring their instrumentation measurements, validating and comparing visual observations with data received from transponders and other electronic instrumentation", the termination of FLT 214 would have been unremarkable.

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#37
In reply to #34

Re: Aircraft Touchdown Accuracy

07/28/2013 8:51 PM

The situation you cited happened to be an incident where the ILS system was shutdown for maintenance and not available! Therefore the pilots in the cockpit are really on their own!

You are quite correct that ultimately the responsibility lies on the shoulders of the pilot! For this same reason it was found according to NTSB the main reason of the crash was due to human errors!

Also, the problem was further aggravated by the pilots in the cockpit are on training, co-piloted by a trainer, although more experience than the trainee, is also a first timer for that type of aircraft!

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#53
In reply to #37

Re: Aircraft Touchdown Accuracy

08/12/2013 1:44 AM

San Francisco is a difficult landing because the final approach is all over water and a shorter runway. Logan airport in Boston is the same. The glide slope and flare are the PILOTS (two of them) number one job, no matter what the aircraft is. They failed pretty badly. Good training and good maintenance have been lacking since the airlines were de regulated way back in the 70's. The older veteran pilots are better at take off s and landings, hands down.

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#35
In reply to #33

Re: Aircraft Touchdown Accuracy

07/28/2013 7:55 PM

Vsar-

re: posts #31 and #33

Your understanding is correct! All that you posted about the ILS system is correct. I have not spent my career with it but my father did, he developed the ILS system.

I was practically weaned on it. I knew what a glide slope and an outer marker were before I knew how to divide numbers. Numerous hours I spent accompanying my father to the test site where the antenna systems were tested for later implementation. Was fortunate to be able to ride in a Bonanza (sp) and a DC-3 during some of these testing runs where I could speak with the pilots about this type of system. They were all very excited about it and spoke highly of it. They would follow the glide slope in and then climb from a very low altitude, quite a thrill!

Your postings are accurate and quite explanatory. Thank you for bringing back many fond memories

Good Luck, Old Salt!

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#36
In reply to #35

Re: Aircraft Touchdown Accuracy

07/28/2013 8:03 PM

His understanding may describe what is supposed to happen on an instrument approach, but VFR were in effect at the time of the crash.

And regardless of what was supposed to happen, nobody was looking at the airspeed and altitude or they'd not have flown into the ground. They did seem to be concerned about the approach heading.

This may help explain the crash:

Were Asiana Pilots Caught In The FLCH 'Trap'?

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#39
In reply to #36

Re: Aircraft Touchdown Accuracy

07/28/2013 9:07 PM

Correct me if I am wrong but in no place in posts #31 and #33 did I see any reference to the accident of Asiana FLT 214. As I read these postings I saw them as clear statements and explanations of some instrument systems available for landing an aircraft. Most of each posting was devoted to the ILS system, one which I am extremely familiar with, having observed much of it's development, specifically the antenna segment of the system.

Yes I agree that the pilot has the ultimate responsibility, just as a captain of a vessel has, but I fail to see the tie in between his posts and the accident you cite. Yes, in that case apparently there was something wrong in the cockpit. What it was we will probably not know definitively until the NTSB publishes it's final report in at least a year.

Where is the tie-in to any previous postings concerning the explanation of these systems as described and the Asiana accident and the circumstances that have been surmised from it?

I'm simply asking a question. I have no interest in becoming involved in a joust of words or anything similar. All I would like is an answer, that's all. As for his explanations, it has been a long time since I came across an explanation of ILS that was so understandable, correct and didn't bury the reader in engineering gobble-de-gook. Thank you in advance for your time.

Good Luck, Old Salt

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#47
In reply to #39

Re: Aircraft Touchdown Accuracy

07/29/2013 11:10 PM

You are right.

My apologies to vsar.

My "instructional moment" went astray from the OP.

Then, I compounded the situation.

And that's why I only fly simulators now.

I'll go sit on the naughty step, if Del's not there.

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#48
In reply to #47

Re: Aircraft Touchdown Accuracy

07/29/2013 11:30 PM

Lyn-

Thank you for posting your message. It is certainly a wonderful experience to see someone post an apology instead of posting a tirade condemning someone's attempt to provide information to the other posters. Too often they also include some sort of questioning of the other's intelligence, whether they have an IQ of less than 45, the supposed lineage of their mother and how it may be related to a canine and other sorts of things. You are to be commended! Keep it up.

Ata' Boy!

For your demonstrated actions to improve this forum I gave a GA .

Good Luck, Old Salt

If you fear a ruin of your reputation, I will withdraw this.

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#49
In reply to #47

Re: Aircraft Touchdown Accuracy

07/30/2013 6:45 PM

Mr. Lyn,

There is really no need for you to apologize, but on the other hand I am glad and thankful you did! By doing so, you just demonstrated some rare and unique characteristic of your personality being a man!

Thank you!

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#50
In reply to #49

Re: Aircraft Touchdown Accuracy

07/30/2013 7:34 PM

Thanks, I've had lots of practice at being wrong over the years.

It comes to me naturally.

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#40
In reply to #36

Re: Aircraft Touchdown Accuracy

07/28/2013 9:15 PM

Mr. Lyn -

About that incident, I think it was too late when the more experienced pilot noticed or felt the problem and to mediate and correct the deficiency. In their desperate attempt, they drastically tried to raise the nose abruptly but forgot the fact that when they do that, the tail end will dip further down and I think that was time it hit or clipped the edge of the runway / embankment!

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#42
In reply to #40

Re: Aircraft Touchdown Accuracy

07/28/2013 10:18 PM

First, no one in the cockpit was flying the 777 for the first time.

It is true that the PIC was making his first approach to SFO in this type of aircraft. He had flown into SFO in different aircraft types before.

The lack of attention to basic air speed and altitude in the cockpit cannot be ignored.

"The pilot at the controls, identified by the airline as Lee Kang-kuk, was still in training on the Boeing 777. He had had 43 hours of flying time in the aircraft, but officials have said he was landing a Triple 7 at San Francisco International for the first time."

I'm not sure anyone was "at the controls" in the classic sense of flying the airplane over the runway threshold, or they might nave arrived there.

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#38
In reply to #35

Re: Aircraft Touchdown Accuracy

07/28/2013 9:03 PM

Thank you Mr. Salt!

I learned the subject matter while taking communications and electronic controls and instrumentation engineering. Its more than 50 years ago and somehow just popped in my mind.

I appreciate your concurrence, it just reconfirmed my memory from time to time although kind of selective is still functional!

thanks again..

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#41
In reply to #38

Re: Aircraft Touchdown Accuracy

07/28/2013 9:17 PM

VSAR-

I appreciate your postings. They brought back many fond memories of a test site in a marsh area parallel to a general aviation airport. Some of my memories are technical but most of them are of my activities there which I now appreciate more than I did them.

That started about 60 years ago as a small boy whose father faced the dilemma of what comes first- work or baby (really child) sitting. He chose both and I now thank him for developing another interest for me.

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#44

Re: Aircraft Touchdown Accuracy

07/29/2013 4:22 AM

Most pilots will aim to plop the wheels on the "piano keys" markings at the near end of the runway.

There's only one problem with harvesting the energy of a landing, and that is the reduction in forward speed effected by such a scheme. If the plane has to take off again to avoid some unplanned event farther down the runway, it needs all the kinetic energy it can so as to become airborne again. Harvesting its energy reduces the pilot's options in such a situation.

Impractical?

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#45

Re: Aircraft Touchdown Accuracy

07/29/2013 10:01 AM

Is the pilot sober?

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#51

Re: Aircraft Touchdown Accuracy

08/07/2013 3:18 PM

A Boeing 737 touchdown is very accurate .

These large aircraft are used extensively as accessible jets in areas where only

limited runways are available , such as northern communities in Canada .

The aircraft can fit into unprepared airstrips less than one mile long .

They use up space getting back into the air , but on a paved airstrip , this is about

a mile of pavement . Cheers ! M. A. Steele , Canada .

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