I don’t have an irrational fear of flying, I do it all the time, but I tend to get a little concerned when I see or feel something unusual. One of the things that especially gets to me is when the plane seems to be taking too long to take off or land. Runways are finite after all. Sometimes I’m on a flight and the pilot hits the runway almost right at the beginning, other times we seem to be over the runway for several seconds before we hit the ground.
So here’s the math question:
Assuming the plane is travelling at normal speed when coming in for landing, and the runway is a typical length for handling passenger jets, how many seconds should elapse flying over the runway before I get concerned? What’s the absolute minimum runway a typical passenger jet needs to land safely once wheels touch ground?
You might message a mod to change your post title to be more descriptive (airport runway lengths and landing speeds). We have several real pilots on the board who could definitively answer your question if they saw it
Sure. I just figured it wasn’t a clear-cut math question. There’s quite a few planes and airports out there, different landing conditions, different countries, different safety areas, etc. all of which call for real-world experience.
Oh, I know conditions vary, I was just looking for a quick and dirty equation using typical conditions, kinda like how you can tell how far away a storm is by listening to the gap between the lightning and the thunder.
Are you sure that attempting to find a rationale for creating your own boundary between justified and unjustified anxiety is quite the best approach to this? In the end, it’s about trusting the airline and the pilot (and/or whatever Almighty you might believe in) - so why not focus on what enables you to do that at all stages of the flight?
While we wait for more definitive answers, I’ll mention two things:
One thing you can observe during take-off or landing are the skid marks from previous landings. (“Either end” only if winds tend to shift direction in that area). The skid-mark density will be a lot higher for a stretch of several hundred feet, maybe a fifth of the way to either end of the runway. This is obviously where most touch-downs occur. If I see that bunch of marks go by and we haven’t yet touched down – or, going the other way, haven’t lifted off – I get just a wee bit concerned.
You mention both take-off and landing in your post, but end up focusing on landing. If I were you, when it comes to runway length, I’d be a little more concerned about take-off. A landing issue can lead to a go-around, which is exciting but routine; whereas a take-off issue, while rare, is more likely to be disastrous, if it’s just the wrong event at just the wrong moment.
What do you mean by “typical”? There’s a huge difference between a heavy jet with 250 passsengers in rows 8 seats wide, and and a light jet with 30 passengers in rows only 2 or 3 seats wide.
In the light jets, I notice that the flight attendant fills out a form and passes it to the pilot: the form is a sketch of the seats, with X’s marked showing him which seats are occupied and which are empty. Apparently the plane’s engines are sensitive enough that it makes a difference. I’ve always wondered if the attendant also has to make a note of how many fat people are on board, and where they are sitting.
Given statistics, I know it’s unjustified. Which is why I wanted to quantify it a little better. Perhaps being above the runway without landing is fine for up to X number of seconds, but if it stretches out longer than that, maybe the very rare could happen?
I did do some of my own research and found that planes overshooting runways on landing usually are pretty survivable events since by the time the plane runs out of runway it’s slowed down significantly. Much better than taking off any running out of runway, at which point you’re out of runway going 150mph.
This isn’t a math question at all, really. I know all the relevant math, but it’s not enough to solve the problem. We need data, which makes this an engineering question.
my “fag-packet” calculation from google yield the following.
Landing speed of a jumbo is about 72m/s
typical runway length for a large aircraft at a major airport is appx 3750m
minimum distance to stop for a heavily loaded jumbo is just over 2000m
Therefore, if you pass the start of the runway and and don’t touch down for 25 seconds you should still be able to stop in time.
To me that sounds like a heck of long time and far more than I’ve ever experienced.
Of course all of the above are just from my idle googling and subject to an incredible amount of variation due to runway length, weather, loading, aircraft type etc. etc. and others more knowledgeable than me will put us straight.
And as someone else has said, you probably don’t want to dwell on it too much anyway. I’m reliably informed that pilots don’t like dying in plane crashes either so I’ll trust their training and sense of self-preservation.
Another “back of a sick bag” calculation (sick bags are more likely to hand in a plane than Novelty Bobble’s fag packet) would be to look out the window on landing and estimate the amount of spare runway when the plane slows to taxi speed (assuming it could if required stop in a very short distance from that speed). Divide that spare distance in metres by the 72 m/s landing speed, and add to that the actual elapsed time before landing. That will give you a ball-park not-to-worry-yet period.
Your phone is supposed to be in “airplane” mode during takeoff/landing, but that doesn’t keep GPS from working. So before your flight departs, fire up Google Maps and go zoom in on your destination airport. This will preload your phone with all the detail maps for that area, including the layout of the airport runways, since you won’t be able to do this in mid-flight. Now when you’re approaching your destination with your phone in “airplane” mode, turn on Google Maps and hold it near the window so it can pick up the GPS satellites. You’ll be able to see exactly where along the runway you touch down, and exactly how far the rollout is before you’ve decelerated to taxi speed. I expect you’ll find that there’s a lot of runway left over.
Also entertaining to install a GPS speedometer app and watch the plane’s speed and altitude during various phases of flight.
Yeah, that’s about what I wanted, thanks. 25 seconds is a long time and it’s sufficiently long that I don’t really need to know the size of the jet. I’ve never experienced a landing where we were above the runway without landing for more than five seconds. Knowing I have 20 more before I should start to wonder makes me happy. The fact that well, maybe I have 30 if it’s a lighter jet or 15 if the jet is carrying a heavier load or the runway is a little shorter than normal isn’t of too much concern when the margin for error is that high. I didn’t know they built in THAT much margin for error, but I guess that’s why flying is the safest way to travel.
In the future, I’ll count to 20 before I get worried, and 25 before I say, to my seatmate, “Is this normal”? And 30 before I scream, “We’re all gonna die!”
IME, that system is adequate for telling roughly where you are on the flight path, but it’s hard to zoom in far enough to tell exactly where you are on the runway when you’re taking off or landing.
Well … strictly speaking it’s entirely possible to fly over the entire runway … touch down on the opposite runout apron … and belly across some farmer’s sweet potato field … and have everyone walk away … just a few years ago, someone executed a perfectly safe landing in the middle of the Hudson River …
Remember … a good landing is one we can walk away from …
Definitely, and as JKelly said, I should be more concerned about bad takeoffs. It’s better to run out of runway doing 60mph than run out of runway doing 150mph.
And the skill of pilots is amazing, although if you watch Air Crash Investigations it seems like it’s always the most experienced pilots who make a mistake that gets people killed. The worst air disaster in history(Tenerife) was caused by the guy who was a safety instructor for his airline! And it really was the most boneheaded error of all time, IMO, caused by impatience and arrogance.
No maths required. You should be landing in the touchdown zone. This is marked by thick white stripes each side of the centerline, with a thicker pair designating the “aiming point”. The longer the runway, the more touchdown zone stripes there will be. The exact layout of the stripes does vary as it depends on whether ICAO markings are used or some other system. Long story short, you should be on the ground at or before the last of the touchdown zone markings. Check the markings on Google maps to get an idea of how many you should be counting.
If you want to do it based on maths, assume an approach speed in the range of 120 - 140 knots and landing roughly within the first 450 meters of runway. 1NM = 1852m. The maths is confused by headwind which will reduce the groundspeed below the nominal approach airspeed.
