View Full Version : Status of Displaced Threshold

Ops and Mops
9th May 2009, 10:18
I wonder if any of the Scottish instructing/examining fraternity can shed some light on a point that has been the topic of some conversation recently.

The short runway at a particular scottish airfield has a permanently marked displaced threshold for obstacle clearance (trees in the undershoot that have since been chopped down). The aerodrome is licenced and the declared distances and runway markings still depict the displaced threshold although the obstacles for which it was put in place no longer exist.

I believe that there is a point of view being given that, unless a flight is being operated for public transport, the displaced threshold can be disregarded and that an aircraft can be landed before the displaced threshold, on the white arrows.

It has always been my belief that, regardless of whether a flight is being operated for public transport or otherwise at a licenced aerodrome, touching down before the displaced threshold is "strictly verboten" and is a reportable incident until the threshold and declared distances are amended accordingly.

Am I being overly sensitive about this, or have I missed something in the ANO or Aerodrome licencing documents that permits a displaced threshold to be disregarded for non-public transport flights? Is there perhaps a local exemption or permission in place that permits this pending the change of the runway markings and declared distances?

I would be grateful for any clarification or pointers to references that may help as I confess to being rather confused by this!

Cheers :ok:

9th May 2009, 10:35
Regardless of the licence status, if the airfield owner has a displaced threshold then you are required to abide by that. If you feel that there is no need to have the threshold displaced then speak with the aerodrome owner / licence holder. Very often thresholds are displaced for mobile obstacles outside or inside the boundary. 999 times out of 1000 the mobile obstacle may not be there. Other reasons can be noise abatement.

Having established that your landing distance including any safety factors permit a safe landing on the declared runway, why would there be a need to cross the threshold at less than the standard 50ft.

If on the other hand, you can make the figures match and you need to be less than 50ft at the threshold then ask yourself what those with 20/20 hindsight would think of this.

An approach to touchdown at a point before the displaced threshold would not be operating in accordance with normal aviation practice and would not be exempt from the low flying rules. Furthremore the insurance company and most likely the CAA would consider such an operation reckless should anything happen.

I honestly can't ever see a reason for intentionally landing before the threshold of a runway.

On the training aspect, a landing before the threshold would be a fail item. Crossing the threshold at an inappropriate height is at best a discussion point.

Look at the figures in the AFM. They are all based on crossing the threshold at 50ft at the correct speed in the correct configuration. Everyone needs to remember that a flat approach crossing the threshold at 10ft can often require a longer distance than a glide crossing the threshold at 50ft and the longest distance of all is caused by pushing the nose down late in the approach and diving at the threshold not to mention the unstable approach aspect.

Remember that most of the landing distance as published in the AFM of light aircraft is airborne distance!!



9th May 2009, 21:57
Well, we have a classic case here.

Someone who doesn't know where to find the answer to a reasonable question posts it here.

Someone else answers, without quoting any references or sources.

The answer is almost entirely rubbish.

PPRuNE strikes again!

Ops and Mops, try calling Aerodrome Standards on 01293 567171 on Monday morning, and asking to discuss with them in the first instance. Ater that, you might need clarification from FOI or GAD (or whatever it's called these days), depending on what sort of operation you're involved in.

And learn your lesson well. Casting around for answers amongst the great unwashed here will never ease the pain of confusion or incompetence!

10th May 2009, 10:30
Relevant regulations include:

Art 126 (1) An aircraft to which this paragraph applies [PT and instruction] shall not take off or land at a place in the United Kingdom other than:
(a) an aerodrome licensed under this Order for the take-off and landing of such aircraft; ...
and in accordance with any conditions subject to which the aerodrome may have
been licensed or notified....

Rule 39 Visual signals
39.—(1) Subject to paragraph (2), the commander of a flying machine on, or in the pattern of traffic flying at, an aerodrome shall—
(a) observe such visual signals as may be displayed at or directed to him from the aerodrome by the authority of the person in charge of the aerodrome; and
(b) obey any instructions which may be given to him by means of such signals.
(2) The commander of a flying machine shall not be required to obey such signals if it is inadvisable to do so in the interests of safety.

Rule 40 Movement of aircraft on aerodromes
An aircraft shall not taxi on the apron or the manoeuvring area of an aerodrome without the permission of either—
(a) the person in charge of the aerodrome; or
(b) the air traffic control unit or aerodrome flight information service unit notified as being on watch at the aerodrome.

My conclusion would be that the use of such undershoot for flights not requiring a licensed aerodrome would be subject to the permission of the aerodrome owner, but not, in itself, illegal.

14th May 2009, 09:49

Can you think of any example of normal operation where you would need to land before the threshold?

If you were an aerodrome owner who designated a threshold for safety reasons would you then ever permit any flight to land before it?

What would your legal position be if an aircraft can't land at your aerodrome because the declared runway is not long enough so you give permission for them to land before the threshold and they clip the hedge on the way in or have any other form of accident in the undershoot (area before the threshold) followed by them passing on a copy of your "permission" to their legal team / your insurance company?

Purchase a big field in the middle of nowhere, plough it up and then land your piston hotship on it. Noting illegal there.............other than the reckless way in which the operation was conducted. Of course if you don't care about breaking your aircraft, you don't claim any insurance and you don't injure yourself then you can do it again and again until you grow tired of breaking aeroplanes!!



14th May 2009, 10:45
Can you think of any example of normal operation where you would need to land before the threshold?

I can't think of a "normal operation" where I would need to land on other than a hard, licensed runway, but I'm given to understand that people do that every day without dying and without breaking the law.

14th May 2009, 12:27
Nice sidestep!

Unfortunately, for your answer it is quite normal for certain aircraft types to land on grass, sand, and even snow during both private and public transport operations.

Still can't say that it is normal for an aircraft landing on a runway (regardless of surface type) to land before the start of the runway.



14th May 2009, 19:58
Sorry to pee on your bonfire again, DFC, but I have regularly landed a twin-jet before the threshold at a licensed aerodrome in the UK, with full approval of the airport operator.

Bookworm had a stab at naming some relevant regs, and didn't do badly. However, this is one of those matters which I would consign to my short list of questions for extremely able command course candidates who need to be challenged. There are legislative aspects and practical considerations, there's custom and practice - and history; there's even room for some personal preference.

I would not land a public transport flight prior to the marked threshold, but when flying privately, I'm very happy to do it with accuracy (and the land-owner's permission). It's good for my heart rate, and the brakes!

14th May 2009, 20:48
The licensed runway starts at the threshold markings, so for public transport or anything else requiring a licensed airfield you must abide by it.

As has been stated above, for a private flight, be it in a 747, a business jet or a piper cub you can do what you want with the landowners permission...the portion of the runway is no diffent to a farmers field.

The displaced threshold is probably there for a good reason, such as not annoying the neighbours, so normally if the airfield operator wants you to comply with it you would, unless they give you permission to do otherwise.

15th May 2009, 02:03
I think the point in the question is that the displaced threshold was there for obstacles that are no longer present. So, what is the continuing purpose of the displaced threshold if the limiting item is no longer applicable?

Following on from that are further questions:

To whom do the threshold rules apply, and when? and,
Is it ever permissable to define your own threshold if the operating surface is suitable before the 'irrelevent' threshold?
What approach gradient is the threshold (displaced or otherwise) predicated on? and finally,
What approach gradient are you required to use for your operation?

Some of theshort runways I operated into on public transport flights in the UK were never affected by a displaced threshold - there was not enough room in 381m to allow for one! Even with obstacles on short final that were best suited to a techniqe of 'dropping on' for the last 50' or so instead of maintaining a constant gradient.

16th May 2009, 11:22
The licenced runway might be one length, the displaced threshold for landing in one direction might give a second length, and then the displaced threshold for landing might give another.

For example look at Farnborough.

The condition of the surface is also relevent, the unused tarmac might be bad condition, and as such usable for an accelerate STOP, but not approved for regular use.

I have also seen raised runway end lights marking the displaced threshold.

Thats all folks...

18th May 2009, 18:36
Runway: -
A defined rectangular area, on a land aerodrome prepared for the
landing and take-off run of aircraft along its length.

Threshold: -
The beginning of that portion of the runway available for landing.

If the reason for the displaced threshold is obstacles that are no longer there and no other reason exists for the displacement then the owner should think about changing the position of the threshold.

However, no matter what the owner says - if one lands before the threshold then one is landing prior to the portion of the runway available for landing. That is not normal aviation practice even if some people do it.


One has to also think about rule 5. Nothing illegal about landing on a beach with the appropriate permission. Rule 5 though still applies with regard to anyone or any vehicle etc on the beach. I.E pass 300ft over someone while landing on the beach and they can report you under rule 5.

The same would apply if you were landing before a marked threshold.


So the JAR-OPS factors precluded commercial ops and your figures were so tight that even without safety factors you felt the need to land before the threshold. I can hear the 20/20 hindsight tooth sucking that will resound should you ever have an incident while doing that!!

To go back to the original point I made -

If you land prior to the threshold on any Skill Test or LPC, you will fail. Those who can't see a problem should try it and see how far they can argue their case!



dont overfil
18th May 2009, 19:15
What are your thoughts on using the bit before the displaced threshold to start the takeoff run?

18th May 2009, 19:37

You've done it again...

Having quoted a 'runway' definition, would you care to tell us which flights require a 'runway' as formally defined?

What is, and is not, 'normal', is not in your gift to define, I'm afraid.

Your comprehension of Rule 5 is also several sandwiches short of a picnic. The '500ft Rule' is not the one to worry about in this respect, in the UK.

Where did I say the runway was too short for CAT? Nowhere, because it isn't. However, the threshold is displaced for reasons irrelevant to my particular operation in that particular aircraft type, so when the law allows, I save brake wear by landing earlier.

Really, you ought to stop peddling your nonsense.

19th May 2009, 10:23
What are your thoughts on using the bit before the displaced threshold to start the takeoff run?

This is quite common and normal. look at the definition of threshold - The beginning of that portion of the runway available for landing.

Take-off can be commenced from the start of the runway - see definition , or from the starter extension if one is provided.



I still can not see a reasonable reason for you needing to land before the threshold.

You say that the published landing distance was suficient to permit commercial operations with your twin-jet. That means that your aircraft could land and stop within 60 percent of the landing distance publsihed in no wind if it is the only runway or in 50% of the headwind and 150% of the tailwind. All based on crossing the threshold at 50ft or 35ft if you are completing an approved short landing operation.

Even if you were to take it to the extreme and touchdown 1cm after the threshold, provided that the figures are as you say, I can not see how you would have to use excessive braking in order to stop in the available runway.

However, on a more serious note, since you specified "twin jet" you must be aware of the following from CAP382 - Mandatory Occurrence Reporting.

5.1 Applicability - Any turbine-powered aircraft which has a certificate of airworthiness issued by the

Appendix B - Occurrences to be reported -

b) Take-off or landing incidents, including precautionary or forced landings. Incidents
such as under-shooting, overrunning or running off the side of runways. Take-offs,
rejected take-offs, landings or attempted landings on a closed, occupied or
incorrect runway. Runway incursions.

Landing before the designated threshold is under-shooting.



19th May 2009, 19:13
DFC, the assumptions King...

20th May 2009, 08:14
I know this is about Scottish displaced thresholds but just to add a bit more fun to the debate how about Exeter where the Easterly runway threshold is displaced for single piston only but not marked (to avoid confusing larger aircraft). You can legally fly an SRA or NDB in a piston single to that runway but not an ILS.

Looks to me like there are no real standards on this.

20th May 2009, 10:02
Exeter is a whole bag of spanners just by itself!

They have tried to do something which is probably a good idea. However, the wording used in the AIP and elsewhere to acheive that objective is wrong.

There is no such thing as an "unmarked displaced threshold". When displaced, the displacement has to be visible to aircraft on approach.

Taking the 08 Exeter example, they are trying to do what many aerodromes in Europe and elsewhere do. They want single engine piston aircraft who do not need the full runway to touchdown well after the threshold. This places them higher over the obstacles prior to the threshold than would be the case if they used the standard aiming point and approach angle.

Unfortunately, every other aerodrome that uses this procedure specifies the "touchdown point" / "aiming point" for single engine piston aircraft. The threshold remains in the same place but the distance from the threshold to the SEP aiming point is greather than the distance to the standard marked aiming point.

ILS and PAPI can only be used when the standard threshold and aiming point are available because this is where they are taking the aircraft to.

Since SEP on 08 at Exeter are using a different aiming point then while making a 3deg approach to that point they would always be above the slope indicated by the ILS / PAPI. Or looking at another way, following the ILS / PAPI would take them too low on the approach and defeat the whole objective of having the separate aiming point.

Having said all that - landing well beyond the threshold is totally different from landing before it and brings up a whole different set of issues.



11th Jun 2009, 10:16
At the Scottish Aerodrome to which the original post refers (I think!), if you do an initial CPL skills test with one particular CAA examiner, observing the displaced threshold results in criticism (at the very least) during the debrief. Words to the effect of "what's wrong with the rest of the tarmac young man" are often reported. Common sense must be applied once operating outside the Public Transport regime.

12th Jun 2009, 12:45
The purpose of the CPL skill test is to assess the applicant's ability to plan and conduct a Public Transport Flight (simulated).

Landing before the designated threshold would be a clear example of how not to conduct a Public Transport Flight!

Perhaps the Examiner was referring to the number of metres bewteen the threshold and where the aircraft touched down.



12th Jun 2009, 13:10
And we could spice up this thread by also discussing the licensed airfield in the south-west which regularly parks aircraft on the runway, before the start of the displaced threshold. Am I allowed to make an approach to this runway, since it is occupied? When departing, can I line up alongside the parked aircraft (assuming the runway is wide enough) to use the entire TORA?

I think, so long as you are happy that what you are doing is safe, the only people who are likely to care in most cases are the insurance company after you have an accident......


12th Jun 2009, 14:19
Of course there is airfield-owners that let some operators land before the displaced threshold in normal operations, just have a look at bremen international airport, for most operators it is 2030m or something, but for the airbus-factory flights its much longer, allowing them to use the displaced thresholds in both ends.

12th Jun 2009, 21:24

What you describe has been approved by the DFS and is published in the AIP. It is clearly reserved for those aircraft specified in the AIP.

People here are talking about landing before the only published threshold or that part of the runway designated by the operator as being the threshold.



12th Jun 2009, 21:37

And we could spice up this thread by also discussing the licensed airfield in the south-west which regularly parks aircraft on the runway, before the start of the displaced threshold.

Was there not a NOTAM dealing with a) the obstacle, b) the change in runway dimensions?

Is this still the case? Let us know where you are talking about.



14th Jun 2009, 22:37
I was hoping someone else might chime in with the name of the airfield, DFC, but they haven't! It's Dunkeswell. No, it's never been NOTAMed as far as I know. And, to be fair, there haven't been any obstructions the last few times I've been, and I'm not a regular enough visitor to be able to say whether this is a change in policy or a coincidence.

However, if you view the airfield on Google Maps, you can clearly see an aircraft getting airborne from runway 05 (the shadow of the aircraft shows that it is airborne), whilst another aircraft is parked at the end the runway, in what is the displaced threshold for runway 23, next to the refuelling area. It's too late at night for me to go onto the AIP to check this, but I believe the area where the aircraft is parked is part of the TORA and LDA for runway 05, and is also part of the TORA for runway 23, and my past experience is that it hasn't been uncommon, in the past, to find aircraft parked there.

A lovely airfield in every other way. Good cafe (for the rare occasions I have time to stop), nice runways, cute blonde working behind the desk. ;) A lot of my CPL students don't like it because it's uncontrolled and they have to mix with some much slower traffic - but it's all good practice, and keeps them very busy!

None of this helps in the slightest with the original question.....


15th Jun 2009, 13:55
Good Spot FFF.

I thnk that the problem is that the aerodrome markings do not accurately reflect the true situation.

The AIP is quite clear with regard to TORA, TODA and ASDA for 05.

It seems likely that if one starts at the start of 05 and measures the declared ASDA and then add on a further 30m for the "strip". From that point draw a slope of some 5% ( 1 in 20) and I expect that the "object" will be cleared.

You could do similar measurments with regard to the approach obstacle clearance surface and again I am sure that the "object" would be cleared.

Or look at it another way, the road just behind the parked aircraft could be a lot closer to the THR runway 23 without causing any limitations on the operation even if there was a line of double-decker busses parked along it!!

Thus while not ideal, there is nothing wrong per say with an object being left where you see the aircraft parked on google.

The confusion arises where the markings show that the portion of 23 prior to the threshold is fit for movement despite a) an obstacle being parked on it and b) the starter extension only being a small portion of that piece of tarmac.

This confusion is further agrivated by the fact that the AIP includes a statement that the threshold of 23 is displaced.

I think that less confusion would exist if the area prior to the starter extension for 23 was marked in accordance with CAP168 just like runway 12 and 05 are marked and appropriate taxiway markings were provided if this piece of tarmac is to be used as a taxiway.

The operator is very clear however in reminding pilots to ensure that they use the threshold for runway 23.

If aircraft are permitted to park on that piece of tarmac, what markings are in place to show the point beyond which aircraft must not park as to do so would infringe on the runway?

Getting back to your point about lining up beside the parked aircraft and starting your take-off run - two problems - 1 it is prior to the starter extension and secondly the starter extension is PPR and I would expect that the aerodrome operator would make it clear to you where you could start the take-off run from and would also take into account blast effects from your aircraft on others parked prior to the starter exctension.

As for NOTAM - not required.



24th Jun 2009, 13:36
Going back to the original topic, as the aerodrome is licenced, would I be correct in thinking that to 'officially' reposition the threshold, a new CAP232 aerodrome and obstacle survey would be required ?

If so then it is probably the cost and red tape involved in that process that is causing the airfield owner to keep it as it is. No doubt an urgent operating requirement would force the issue.

24th Jun 2009, 14:39
SimJock:would I be correct in thinking that to 'officially' reposition the threshold, a new CAP232 aerodrome and obstacle survey would be required ?They should be doing an annual check survey anyway. If they haven't done that they would have to re-do the full AGA survey to re-height the relevant obstacle.