Extended Thresholds
 

I have noticed that on many runways, the threshold markings are some distance from the actual start of concrete. Rwy 24R at Manchester, for instance.

This reduces the Landing distance.

Why is this? Anyone know?

Wouldn't it be safer to have the WHOLE runway available for landing?

swashplate

sometimes runway thresholds are displaced because of obstacles along the landing/takeoff path. Othertimes that very section of pavement is unsuitable to bear the impact of a landing aircraft, so it's used, sometimes ,just for giving extra distance to aircraft aborting their take off.

This brings up a related ops eng problem.

Sometimes it may not be entirely obvious just from where TODA/TORA/ASDA commences. The prudent operator and pilot make appropriate enquiries in cases where the runway is critical. I very nearly got caught out assuming when I ought not to have done so with a runway at a particular Australian airport. In that particular case, the go distances commenced from a point which was, in my opinion, not at all intuitive.

BD tells is like it is in that the usual reason for a longterm displaced threshold is to provide for the obstacle clear surface appropriate to the runway code.

Usually there's an obstacle intruding into the approach gradient. The solution is usually to 'jack up' the approach path to provide a certain, defined amount of clearance above the most critical obstacle.

The result of raising the entire approach path is that it intercepts the ground ie the runway, further away - so that's where the beginning of the useable part of the landing runway (in that direction) is marked.

If this were not done, pilots would aim for the marked threshold at the beginning of the tarmac & be at risk of colliding with the obstacle.

Each runway useable in both directions has 4 separate obstacle clear gradients calculated for it:

1. Obstacles after take-off in one direction.
2. Obstacles on the approach path in that direction

3. Obstacles after take-off in the opposite direction.
4. Obstacles on the approach path in this opposite direction.

With the various buffers that are applied this determines the minimum gradient that must be achieved to clear the obstacles safely.

There are limits on just how steep an approach or departure path can be - hence having to move the threshold or reduce the available length for take-off.

Some runways use special procedures to circumvent particularly awkward obstacle clearance problems eg London Docklands has an unusually steep approach - so much so that pilots AND aircraft have to be certified to operate into the airport.

NB: I think London Docklands also has an element of noise abatement in it?

Other runways may have a kink or bend in approach path eg Kai Tak, or have the final approach be slightly offset.

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Any errors are entirely my own but at least they are mine.....all mine......mwah ha ha ha......

Oy...who are you chaps? Why the funny white coats? Can I have one?

LET GO OF ME!!!

"so much so that pilots AND aircraft have to be certified"

Do you mean certificated, but in the case of LCY perhaps certified is the correct term!

Quite so.

I've never understood how/why 'certificated' is different to 'certified'.

In practical terms, the two words are somewhat similar.

However, they do come from different roots and the verb "certificate" involves the furnishing with, and/or authorising by, a bit of paper called a certificate. "Certify" has a number of meanings, including one similar to "certificate", but generally involves more the intent of formal attestation rather than the formal issue of a bit of paper to frame and put on the wall.

It has been traditional in airworthiness circles (particularly) for far longer than I have been in that game to use the verb "certificate".

Now, if you don't like that, then feel free to use "certify" - it doesn't really matter and, ultimately, becomes a point of preferred style. However, be prepared to continue seeing the other, probably somewhat more appropriate term, used by the trade.