Moderator Note
Since there’s no math question here, I have changed the title from “Fairly simple and stupid real life math question” to “Airport lengths and runway speeds”.
Hopefully this will attract some of our resident pilots and aviation experts to the thread (though I see some have already found it anyway).
Day in & day out, takes about 20 - 25 seconds from full power to rotation unless the airport is very high or it is a hot still day in Denver.
Load and stage length determining fuel load can make a noticeable difference for those that fly big iron a lot.
FWIW, I wouldn’t worry overmuch about how far over or under threshold they’re landing.
How far left or right of centerline is much more interesting. 
Commercial airliners with all engines operating properly are never short on runway length for takeoff. The “balanced field” specification requires they must be able to accelerate to essentially takeoff speed, lose an engine, then (at heavy weight) either brake to a stop or take off successfully on the remaining engine(s). Since most jets are now twin-engined, this means each engine is very powerful.
Likewise upon landing, most commercial airliners have thrust reversers which can greatly shorten the landing run. However the specifications generally require the plane must be able to land without reverse thrust. So if the thrust reversers fail the plane won’t run off the end, it just puts more stress on the brakes. That means with reverse thrust there is almost never an issue about running out of runway – even if the pilot somehow miscalculates the touchdown point or speed. This doesn’t mean nothing will never, ever go wrong, but there is a lot of safety margin built into the system.
During certification each airliner model must prove its ability to perform a “rejected takeoff” at max gross weight. This is one of the most strenuous tests possible and usually melts the tires. Normal passenger service doesn’t come close to this: https://www.youtube.com/watch?v=_g6UswiRCF0
In most things involving commercial aviation there are multiple unseen layers of redundancy and various conservative safety factors like the above. This is why commercial flying is so safe. It’s not uncommon to talk to a retiring commercial airline pilot who has never lost an engine, never had a hydraulic problem or any other major system failure his entire career. That’s the difference between actual reality vs how flying is often depicted.
based on that Info I’ve looked at the Heathrow runways and it looks like the first touchdown marks are about 3-4 seconds flight time from the threshold (about 250-300m), the optimum touchdown point looks like 8 seconds from the threshold (appx 550m) and the last markings are at about 15 seconds (1000m) but even then there is nearly 2700m of proper runway with another few hundred m of taxiway after that. So you could easily fly a jumbo for 25 seconds over the runway and still make a safe (ish) stop.
That sort of feels right to me. Before looking into any of this had you asked me how long it takes from threshold to touchdown I’d have guessed 5-10 seconds max. Seems like that would be the case because the pilots would mostly be aiming for that preferred point.
And I don’t think that Heathrow runways are even particularly long.
Both of Heathrow’s runways are over 12,000 feet long.
Detroit has several ranging from 8,000-12,000.
Denver? All are 12,000, except one that’s 16,000 (but then they had shitloads of space to work with).
Other airports that I looked at all seem to be in the range of 8,000-12,000 feet. IOW, Heathrow’s runways appear to be at the long end of “typical”.
So, that wasn’t a good landing?
and to add… “but a great landing is one where you get to use the plane again.”
Be concerned when you smell fire.
“That was the shortest runway I’ve ever landed on.”
“Yeah, but look how WIDE it is!”
Lots of good isolated facts so far. I’ll try to synthesize it into the bigger picture.
Assuming Boeing or Airbus size & style aircraft …
Approach speeds vary based on aircraft type, weight, flap setting, and wind conditions. 145-180 mph will cover just about any situation, with the lower part of that range being a lot more common. I landed last night with a target speed of 151mph. That translates into about 200-220 feet per second horizontal speed and about 12-15 feet per second vertical speed as we near touchdown.
The touchdown aimpoint is typically 1000 feet from the start of the runway. If we made a carrier-style landing, that’s about where the wheels would impact.
Done ideally, the round out at the bottom should consume about 300 feet. Implying touchdown about 1300 feet down the runway. With a distinctly clunky arrival. We’re driving a dump truck, not a limousine.
More realistically many pilots will burn some runway length (extra 500-1000 feet = 3-5 seconds) delicately feeling their way down the last 3-5 vertical feet rather than deliberately planting the airplane on target. On a long runway (defined in more detail below) this is pretty harmless. On a short runway that can be a bad habit. Add in some snow or ice and it can be a disastrous habit.
Aiming to touch down much closer to the starting end is impractical because the descent angle is real shallow, 3 degrees being nominal. Aiming for the very threshold would involve dragging the rest of the airplane real close to the ground upstream of the runway. As well, in a very big airplane, where you think your face would hit the ground and where the wheels way back there and way down there actually hit the runway can differ by 300 or more feet.
The opposite error is vastly more common, where as you get closer your aimpoint drifts down the runway and passing a couple hundred feet you find yourself aligned to impact 1500 feet down vice the desired 1000. You can’t really fix that in the distance remaining; shoving the nose down abruptly there to repair your goof leads to nasty crunching noises followed by unemployment. So you’re forced to accept the long touchdown or go around if it gets bad enough.
Speed is a similar challenge. You have a pre-computed speed to fly. In calm conditions there’s not much excuse for being more than a couple mph off. In gusty conditions the tolerances inevitably go up, so some pad is added so you’re still safe on the slow side of those larger tolerances.
If you allow yourself to get more than a few mph above target, the round-out and feeling for the runway process can turn into floating along just above the runway for an extra 1000 feet = 4-5 seconds. That is very poor technique. On the other hand, getting slow can result in a stiff impact or even dragging the tail in a nose-high state. So many folks develop the habit of carrying some extra speed because the downsides seem less catastrophic. Which they are until the runway gets short, slippery, or both.
At least in the US, Boeing & Airbus sized aircraft use runways varying from the extremely short 5700 feet at John Wayne Orange County, to the still quite short La Guardia and Washington National at 7000 feet to to the very long 13,000 at DFW and 16,000 at Denver. The extreme length is needed for takeoffs, not so much for landings.
Hope that gives some context.
To directly answer this
About 5 seconds from passing the very beginning of the runway to touchdown would be Yeagertastic, 7 is typical, 10 seconds is sloppy, and 15 seconds is time to go around.
Subject to the caveat that the legal beginning of the runway is not necessarily where the physical concrete begins. So looking out your side window you need to be able to identify the paintwork that represents the legal beginning.
As to this:
From main wheels on, a max effort stop on a dry runway would be in the 2500-3500 foot bracket. Wet or snowy is 50 to 150% longer, big headwind is 10% less.
Late add of one final sentence:
A more typical stopping effort is half again to at most double that.
And that, ladies and gentlemen, is why we have the dope! you need an expert? we’ve got an expert.
How did i know exactly what video that was before I even clicked on it?
The vodka burner is rolling…
Something I have not seen mentioned here that can save your ass in either a failed takeoff or landing is EMAS, an acronym for engineered materials arrestor system. Normally these are 600-1000 foot long areas of frangible concrete at both ends of a runway and behave very similarly to the gravel runaway truck pits on mountain roads. You can walk on its smooth surface, but the weight of an airplane will punch through the top layer and the plane will be quickly stopped with minimal damage.
The intent was to catch planes that can’t stop because the brakes failed, the pilot badly overshot, etc., but the material will also work for a failed or rejected takeoff as long as the wheels are still on the ground.
Huh. I’m surprised – the wheels are so hot after a failed takeoff hard-braking – basically full of flames, barely contained in the tires by careful design, it doesn’t seem like a good idea to mess them up with some deliberately punched-in surface debris.
But obviously they thought of this, so I guess the potential pluses outweigh the potential minuses.
The big plus is the airplane stays inside the airport instead of plowing up Farmer Brown’s corn field or worse, joining rush hour traffic on the freeway. :eek:
There’s that! 
Here is a 757 landing in 2007 at Toncontín Airport in Honduras, which at that time had 5,442 feet of usable runway: https://www.youtube.com/watch?v=vXj65a0490Y
Cockpit view of 737 landing at Toncontin in 2007, including pilot commentary at the end: https://youtu.be/iAxAso8xSo0?t=151
Toncontin is also at 3,297 ft elevation plus it’s surrounded by mountains. In 2009 that runway was improved to be a bit easier.
Hopefully the OP can see that if an airliner can do this, there is a significant safety and performance margin that’s not used under normal conditions and typical runways.