How do pilots calculate their rate of descent so that they’re at ground level when the reach the runway?
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They follow a glide slope at a specific angle. There are lights next to the runway that show if you’re above or below this path, as well as radio location systems, altimeters, and gps.
Mostly they don’t: the plane does that for them. But outside that, they know the distance to the runway and their speed, so they know how much time they have. They know their altitude so they know how far they need to descend. So they know the distance they need to travel and the time needed to do it. That’s the rate of descent.
Pilots learn and practice the combination of airspeed and descent that will get them to the ground at the right time. Some runways have sets of lights running along an edge of the runway. If you’re on the right glide path, you’ll see red & white lights. If you’re low, you’ll see more red than white. Too high and you’ll see more white than red. And if the plane has an autopilot capable of landing it & conditions are nice, the pilot doesn’t have to do much at all.
Airliners and general aviation/business aviation airplanes on an instrument flight plan generally don’t…they file a flight plan and air traffic control tells them when to descend and “steps them down” in altitudes until they’re at the right altitude to do the final approach. Then, when they hit the glideslipe, it’s like u/SoulWager said…they follow that path down at a specific angle. Large aircraft have a dedicated Flight Management Computer (FMC) that takes the whole flight plan and does all the calculations…when to climb, when to turn, when to start descent, etc. The pilots just have to follow it, subject to air traffic control commands.
If you’re flying visual (i.e. air traffic control isn’t telling you exactly where to go) then there’s a particular altitude you want to enter the traffic pattern, usually 1000′ above the runway. So you use basic math to figure out when to start descending, like u/musicresolution describes, and you want to be at that altitude *before* you reach the airport area. Then you fly a standard traffic pattern; part of that for any airplane is learning where and what throttle setting to put in so that you lose that 1000′ to the runway in the right distance.
Lol who else heard the 50 40 30 20 in their head?
A few ways:
On visual landings, there are 4 lights next to the runway which are different colors depending on the angle you view them from. They should be two red, two white. Four reds mean you’re too low, and four whites mean you’re too high.
On instrument landings, the instruments tell you too high or two low. This is also based on your angle relative to the runway.
Airport maps will feature local landmarks and instrument radio beacons plus their distance to the runway. If you know your altitude and distance from the runway, you can quickly figure out how many feet per minute you need to descend.
What I am about to describe is true for most modern airliners.
From cruise to the beginning of the approach we use something called VNAV which is short for Vertical Navigation. The flight management system calculates a descent with ideally idle thrust. We input the wind and the FMS more or less does the rest. Quite often ATC will screw up our plan though by not letting us descend when we want to. In that case we can use our judgement or a few other predictive tools we have. Based on our current ground speed and descent rate most modern aircraft will be able to predict (and display on the map) where we will reach our target altitude.
Now, once we are on the approach (usually 10-15 miles from the runway and around 3000 feet above the ground) we will typically use either an electronic glide slope from a ground based navigation aid installed next to the runway, or we can use a GPS-calculated glide path from the FMS. Not all planes are capable of this but most modern airliners are.
Additionally, many runways have a visual glide slope aid called either PAPIs or VASIs. As others have mentioned these are four red and white lights that tell the pilot if they are too high or too low. Four white lights means we are too high. Two white and two red means we are on the right path. Four red means we are too low.
Since nobody else has mentioned it. When planning for when you should be starting your descent, a general rule of thumb is the “3 to 1” rule. For every 1,000ft you need to loose, you need 3 miles. So if I’m at 30,000 ft and I want to be at 3,000ft 10 miles from the airport, I will need 81 miles to descend. + the 10 from the airport so 91 miles from the airport I want to descend. (30,000-3,000= 27,000) (27×3= 81) (81+10= 91)
So if ATC hasn’t started me down by then, I’ll go ahead and request lower.
The airplane does this for us nowadays with the magic of VNAV. However some basic rules of thumb are still used for manual descent planning. Common “3 to 1 rule” means that for every 1000 feet we need to lose it takes 3 nautical miles of distance traveled at a normal descent rate.
The reality is outside of using vnav a lot of it is just guessing. Nobody these days does any complicated math to calculate a perfect descent rate. Usually it’s just looking at our displays and seeing “oh shoot we aren’t coming down fast enough” and then doing things like increasing airspeed or using speed brakes to increase the descent rate until it looks right.
Once on final approach as others have said we then either are using vnav, an ILS glideslope, or visual lights to descend perfectly to the runway. There’s tables available for your ground speed and the optimal descent rate for a particular glide angle (normally 3 degrees) but nobody really uses them these days because the descent rate rarely changes all that much.
When I fly VFR I’m looking for the spot in the distance that isn’t moving relative to the horizon. That is where the plane will touchdown if the trajectory doesn’t change. I adjust the pitch and power so that that flight path arrives right at the runway. I also account for flaps as necessary
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