Displaced threeshold

[Fuji Abound]

I seem to remember at my local field when a significantly displaced threshold was introduced the flying clubs got very excited about landing before the threshold and notices were posted about ATC making mandatory reports to the CAA. How do people deal with displaced thresholds and what strictly is the legal position?

[Lew Ton]

Is the airfield licensed? If so, touching down on the bit before the threshold could mean you using a bit of unlicensed airfield. Not itself a problem, per se, except the airfield owner/operator may have rules about it, and not all aircraft owners permit their acft to use unlicensed airfields, or parts thereoff.

Why is the threshold displaced? Could be a licensing issue if it is to provide obstacle clearance on the final approach. Or may be for noise, to keep acft higher, in which case it is purely a matter for airfield owner/operator.

In any event, I would not have thought it comes within the realm of mandatory reporting, unless there are other safety-related issues.

[Edited for typo.]

[This message has been edited by Lew Ton (edited 17 June 2001).]

[FurryDice]

If they say land at the threshold, then why not? You could open a real can of worms if you landed before the threshold, and you had an incident.

FuzzyMan

[saggy]

A friend of mine doing his QXC landed before the threshold at one of the landing 'away' aerodromes. As a result, they wouldn't sign him off...he had to do another circuit to prove that he could do it!! I'm guessing that's procedure http://www.pprune.org/ubb/NonCGI/redface.gif

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G-XX finals and ...hopefully land! :)

[Lucifer]

Usually due to obstacles: if you land before the new mark, you are too low: you should have gone around.

[Rusty Cessna]

Interesting how some people get the impression a displaced thresh hold is there to make the runway look pretty.

In my experience they are there mainly for obstacle clearance or to bring the runway in question into a set of dimensions that make it legal.

It's there for a reason so use if in accordance with that reason. I'm sure that if something went tits up on part of the runway before the displacment you would be liable for any damage, and it could cause everyone involved big hassle.

Just my threpence, by the way, this is not intended to get at anyone in particular.

Rusty.

[Spoonbill]

Displaced thresholds are there for a reason, not as a challenge to see who can land before it, on it or 2 feet after it.
As Rusty says, it's usually an obstacle, such
as a public road or fixed object.
The major problem with not complying with the requirements of something like this is, (aside from doing yourself, someone else or the aircraft damage), that you'll screw things up for everyone else.
Those of you who have the pleasure of trying to appease one of the many NIMBY local authorities in this country will know that they only need one piece of evidence that an airfield is not complying with every rule in the book to restrict or even stop flying taking place.
If you see a displaced threshold, comply with it first and ask questions later.

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It wasn't me.

[Bouncy Landing]

I fail to see how anyone can think it acceptable to land short of the displaced threshold. If you get into such a habit and one day try landing "before the threshold" at another airport you may end up in a field, motorway or even a cliff face (try landing short at Jersey)!

[Bluebeard]

You land on the runway...that's the rule isn't it? If its not part of a runway, don't try and land on it.

I'm sure a few of us may have touched down a smidgeon before displaced thresholds on occasion, but then a few of us may equally have been guilty of the odd tiny CAS or land clear rule infraction, these are equally valid rules which we should aim to stick to.

Not saying I'm perfect mind, far from it!

[squibbler]

A displaced threshold is simply based on the OCH's for the approach to the runway concerned. If you land before it you have technically landed short of the runway. The papis (if available) are calibrated to give the glideslope to threshold gradient , usually 3 degrees. So if you land short you have cocked it up good style. If you do so, reporting action is mandatory from ATC.

But in reality most ATC'ers couldn't be arsed filling out the paperwork so we'd grass you up to your flying club and probably dispense a high minded bollocking over the phone ;)

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No, you can't have 09!

[Legalapproach]

I remember a case a few years ago where the CAA successfully prosecuted an ATPL for negligently endangering the safety of an aircraft when he touched down on the grass about 100yds prior to a tarmac runway. The landing occured in poor viz although the pilot could see and was flying the PAPI's. If landing before a displaced threshold puts you in potential conflict with an obstacle or road etc the same argument could be raised.

[Final 3 Greens]

Squibbler

One should not land before any runway threshold - period ... but I am interested in why you quote PAPIs in this instance.

If a light single under VFR is making a typical 5-6 degree approach, they are surely irrelavent?

3 degrees works well with heavy metal speeds (and redundant power), but is rather on the shallow side for a light single, especially if there is obstruction creating a displaced threshold ... e.g. for a PA28-161 (source OATS expanded checklist) at vref +5 (70 kias) the profile would be circa 300' per NM.

Therefore an engine failure at say 1.5 miles out would not allow many options. 450 feet of energy wouldn't make the runway (best glide 75 kias, sink rate say 600 fpm = 480' per NM - if the pilot was very accurate and quick to react)and the best option may be very unpalatable.

Do many experienced PPLs fly the PAPIs with a in your experience?

Apologies to all for wandering off thread, but this matter does intrigue me.

[Bouncy Landing]

Always do use PAPIs (and VASIs - showing my age!) when they are available, instructor always taught me to use them, even for basic circut work and later below decision height on IMC training.

Surely we should use all legally available aids to aid a safe landing in the correct location from the correct glidepath?

[Rod1]

If I am flying VFR, and have no noise abatement considerations, I always try to keep my circuit within gliding range of the airfield. I do this because I only have one engine, and if it stops and I am outside of gliding range, I am in big trouble.

I do not know many single engine aircraft that could glide to the runway from a 3 degree approach, so you better hope the undershoot is good. Why take the risk?

On the displaced threshold question, I note several postings do not consider the area before the threshold to be part of the runway. I assume in this case they do not use it for takeoff either? What do people do if there are no runway markers, this is the case on some unlicensed strips? If a displaced threshold is set for a 3 degree slope and you are coming in at 7 degrees, you know this is the case and have checked with the airfield operator and got his permission to use the undershoot in order to avoid an overshoot through the hedge, would you still not use it?

I put the following forward because the topic is much more complex than it at first appears. I do not normally land before a displaced threshold, but I have done it, quite deliberately, on a number of occasions.

[Final 3 Greens]

Bouncy Landing

If you re-read my posting, you'll note that I specified VFR approaches, as instrument approaches are made using a different profile(s)and aids such as PAPIs and lead in lights.

If you play under IFR, you must accept the different risk profile that comes with the approach being undertaken.

Under VFR, one has more options - your choice PAPIs, mine steeper approaches - both perfectly legal - in the end "you pays your money and takes your choice!"

Rod1

WIth regard to displaced threholds on licensed airfields, the TORA and LDA are the legal disctances, so usually the TORA includes the displaced threshold at the starting end, therefore the TORA is often longer than the LDA for a given runway.

[squibbler]

Final 3 Greens,

The papis, as I'm sure you are aware, are there to indicate a 3 degree glideslope to the threshold. They are simply a guide for VFR traffic whereas for IFR traffic they are an integral part of an instrument / visual approach. If you want to fly VFR 5-6 degrees on approach that's your perogative, but on an approach that steep you'd find it a bit difficult to land before the displaced threshold; well wouldn't you?

Incidentally, you completely lost me shortly into the 3rd paragraph, I'm just a lowly ATCO with his feet firmly on the ground! I did however do 15 hours training in a C150 as part of my ATC training (solo after 9 hours) and my instructor taught me always to use the PAPIS.

Squibbler




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No, you can't have 09!

[Final 3 Greens]

Squibbler

Sorry - didn't mean to faze you! Your comment was spot on and I assumed that you were an ATCO with a PPL; btw I don't buy into the "lowly" ATCO statement, we're all on the same side!

5-6 degrees sounds steeper than it is in a light single; its the same angle that is used at London City and whilst it looks impressive for a 146, a PA28 or Cessna can manage it easily.

As a rough rule of thumb that you might find intresting, to make a 3 degree glideslope halve your groundspeed and add a zero - so approaching at 70kts, 350 fpm would be required (or 300 feet per NM... divide by g/s and then multiply by 60)

To hit a 6 degree slope at , this figure becomes 700fpm/600per nm, which you may recall from your C150 days is circa a glide approach. Obviously, the stronger the headwind component, the less the rate of descent required to maintain the glidepath.

The PPL syllabus focuses on powered approaches for normal landings which is why your instructor will have taught you to to use the PAPIs in your training, but many of us use steeper angles to add a safety margin in case the engine has problems near the filed; unlikely, but it all depends on one's personal view of risk!

I would not land before a displaced threshold - if the field is not long enough, pick another one or another aeroplane with appropriate field performance!

[Fuji Abound]

Having posted the question I was very interested in everyones reply. At the airport I had in mind there is a railway line that crosses the approach. A landing before the threshold from a 3d glideslope would result in passing close to any trains on the line. I agree with others sentiments that a 3d approach, or flying the PAPIs, in most singles would result in it being impossible to make the threshold in the average GA single after an engine failure. What you decide to do is of course up to you and your risk assessment of an engine failure on the approach! Flying a much steeper glideslope may mean a landing before the threshold with solid clearance of the railway line in this case entirely feasible, although why you would perhaps want to land before the threshold is another issue. Part of my question was whether it was legal to do so, and whether in fact the CAA would take any action - I am still not certain of the answer. As a by the by I have noticed at some other airports with displaced thresholds with no obvious reason (I am sure there is one, just not apparent to me) the pre-threshold bit is sometimes in poor condition which for other reasons may also make it unwise to land pre-threshold. I agree that the rules would change IFR.