Final glide, and how wrong it can be
Final glides are not lost to arithmetic. The arithmetic is one division and it is never the thing that goes wrong. They are lost to sink nobody forecast, a tailwind smaller than it looked, water somebody forgot to dump, and starting before the numbers were real. So this page gives you four heights rather than one, and spends most of its space on how far they move.
The glide, in four numbers
Height you have
Minimum
flown at
Planned, at MC
flown at
Worst case
all three errors at once
Sink it absorbs
beyond what you forecast
This is a height budget, not a terrain-clearance check
It assumes a straight glide over ground you have already assessed, and it knows nothing whatever about what is under the track. Rising terrain beneath a final glide is a classic accident chain, and an arrival-height floor at the goal does not protect you from a ridge at forty kilometres. Nor does this page know where your landable fields are.
It is a briefing-room demonstrator, not a planner. Fly the aircraft and the computer in front of you.
Height needed against distance
Where a line crosses the height you have is the furthest that case reaches. On these numbers the planned glide gets you , and slowing to best glide over the ground gets you .
Why there are two heights and not one
The planned figure is the height needed flying at your MacCready speed. It is not the least height that reaches the goal. That one is flown at the speed which maximises glide ratio over the ground — the MacCready-zero solution with the wind in it — and it is always lower, here by .
That gap is the reason this tool exists rather than a refinement of it. A pilot told "not achievable" at MacCready who would in fact arrive by slowing down has been given the wrong answer in exactly the situation the number was consulted for.
What actually costs you the glide
The glide
What you want left over the fence, after which you still have to fly a circuit.
Where you are
A pilot on QFE set to the destination and a pilot on QNH type different numbers into the same box, and both are right. Say which you are on.
Height above the goal:
What the speed in hand is worth
Arriving fast means arriving with energy you can trade for height, and it is routinely overstated. From your cruise of down to an arrival at best glide, the recoverable trade is .
The figure people quote is V²/2g, which here would be . That measures kinetic energy against zero airspeed, and you do not arrive stopped — so the usable trade is less than it suggests. Counting the larger number as available final-glide energy is an optimistic error in a safety-adjacent place.
The polar
A standard-class curve at 33 kg/m², here so the page works before your manual is open. It is a worked example, not a claim about any particular aircraft — put your own numbers in before you believe a height.
Read three points off the polar graph in your flight manual, and make one of them fast. Every final glide is flown fast, so a curve fitted only to slow points is extrapolating over precisely the range this page depends on. That is also why the two-number entry mode offered on the polar tool is deliberately absent here.
What this assumes, and what it can't know
This is ground school, not a flight computer. It exists to show how far a height requirement moves when the air is not what you were told, which is a thing worth knowing on the ground and a bad thing to be reading on a phone at 700 metres. Fly the aircraft and the instruments in front of you.
Repeating the caveat above, because it is the one that matters: this is a height budget over ground it knows nothing about. No terrain, no airspace, no landable fields.
Steady air, steady wind, one straight leg. The wind is a single constant component along the track. Real gradients, a wind that veers with height, and a track that bends around airspace are all outside the model, and all of them make it worse rather than better.
A quadratic polar is a fit, not the physics. Beyond the range it was fitted over it under-predicts sink, because real drag rise outruns a parabola. Since final glide lives at the fast end, that error flatters the glide. The bugs slider is cruder still: one multiplier on the whole curve leaves best-glide speed unchanged, whereas real contamination would lower it, so the modelled height requirement is too low. Every simplification here points the same optimistic way, which is why the worst case deserves more of your attention than the planned figure.
The worst case is not a probability. It is what happens if all three of your chosen errors turn up together, which is neither impossible nor likely. It is a bound to think against, not a forecast.
Not yet reviewed by anyone but us. If you instruct, or one of these assumptions is wrong, we would genuinely rather hear it than not — tell us and we'll credit you here. This is ground school, not a flight computer, and not an authority on your aircraft. Fly the numbers in your own flight manual and the instruments in front of you.
Where these numbers come from
The tangent construction behind speed to fly, why ballast never changes your glide ratio, and what a polar can and cannot tell you.