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andreadebiase

Where is the correct touchdown point?

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little confused here...depending where i am looking I feel like i get two different answers so here is why i am posting this, the question is, should your wheels touch the asphalt past the aiming point (purple oval) or before?

 

this figure

 p_runway.jpg

suggests that the L area is where your wheels should touch the asphalt, so at or after the aiming point (purple oval) but the below figure has the "touchdown zone markings" before the aiming point.

 

I assume that anywhere after the threshold is ok but where is the correct landing spot, before of after the 2 aiming marking? 

 

runway_aim_touchdown_markings.png

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According to the FAA Airman Information Manual (AIM) the touchdown zone is the first 3000 ft of the runway.  If equipped with touchdown zone lighting, they will extend from 100 ft to 3000 ft or runway midpoint.

 

The touchdown zone markings as shown in your bottom illustration extend from 200 ft to 2500 ft, do not confuse with the touchdown zone.

 

Realistically, you use the aiming point to gauge your descent and it generally coincides with the glideslope and maybe the VASI or PAPI (although they are often not coincidental).  Find a bug spot on the windshield and keep the aiming point from moving lower or higher until flare.  Your flare will usually put you wheels down beyond the aim point, hopefully well before the 3000 ft point.  If you have a short runway you may be aiming before the aim point marking but it is still a good reference, just don't undershoot.  Be aware of wind conditions that might contribute to a rapid sink just before flare, which is why we are not aiming for the threshold.

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What Don wrote (but it's actually a bit more complicated than that, especially on runways shorter than 3000 ft, which are not a factor for 737).

 

So, you may be wondering *why* not just aim for the threshold? I know I was... The reason for that is because having a "displaced" aiming point, gives you an extra safety buffer should be undershoot it :-). Same reason why your aiming point is mid-field when trying a deadstick landing.

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I watched a video of an Alaskan bush pilot land his super cub on the threshold and make an exit at the numbers... wind was almost the same speed as his approach haha... it was great.  It was on the Discovery series Flying Wild Alaska.

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To add to what Dan said for my airline and I'm pretty sure it's the same for the others it's the first 3000 feet or the first third of the runway whichever is shorter. For example I mostly fly into Maui (PHOG) which is 6995 feet. 3000 feet is more than the first third so we have to be on the ground by 2331 feet when we land there. 

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It's also worth pointing out that your landing performance is predicated on crossing the threshold at 50ft and touching down at the appropriate point (i.e around the 1000/1500ft markers). There is no need (unless, as mentioned above, you are flying a small STOL aircraft in the bush) to have the wheels on the ground on the numbers -- you are just increasing the risk of an undershoot and your landing performance (which is already conservative) takes the threshold crossing height in to account.

 

Bear in mind that in a large jet the main gear is generally a considerable distance below the pilot's eye height: the B747 is particularly extreme because of the high flight deck, with the main wheels passing some 45ft below the pilot's eye height in the approach attitude. This means that in a 747, if you put your eyes 50ft over the threshold your main gear will be just 5ft off the runway!

 

This is something to be mindful of when using visual glidepath indicators such as PAPI lights, because the PAPI guides the pilot's eyes to a particular height over the threshold. Normally the PAPI will have been calibrated to take in to account the eye-wheel height of the most common types of aircraft using that runway, which means that if you are flying something atypical (say, a B747 in to a regional airport that normally doesn't see anything bigger than a CRJ, or on the other end of the spectrum a C172 in to a large international airport) the PAPI lights may not be appropriately calibrated and therefore should be disregarded in the latter stages of the approach (typically below around 200ft). In the B747 it is fairly typical to fly 3w1r on most PAPIs to take in to account the greater eye/wheel height.

 

Likewise, in the B747 one would normally aim for the 1500ft markers (rather than the 1000ft markers) for the same reason: you need to put your eyes higher over the threshold (by aiming further down the runway) to ensure sufficient main gear clearance.

 

The B737 eye-wheel height is only around 20ft, so much less significant, but still very much non-zero!

 

(NB: the radio altimeter is normally calibrated so as to read 0 at main gear touchdown, whichever type you are flying, so you should hear "fifty" as you go over the threshold).

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