Type the question into a search engine and you get a number. A 747 needs 10,000 feet. Or 3,000 metres. Or 8,000 feet. The numbers disagree with each other, none of them cite anything, and all of them are answering a question that does not really exist.

There is no single runway length for a 747, because runway requirement is not a property of an aeroplane. It is a property of an aeroplane at a particular weight, on a particular day, at a particular airport, on a particular surface. Change the temperature by fifteen degrees and a 767-200 needs nearly two thousand more feet of concrete.

What does exist is a set of published charts from Boeing and Airbus, and a standard FAA method for reading them. Work through that properly and you get real answers — including one that catches almost everybody out.

The numbers, from the manufacturers' own charts

When an airport authority needs to know how long a runway has to be, it does not search the internet. It opens the manufacturer's Airplane Characteristics for Airport Planning manual — Boeing and Airbus both publish them free — and follows the five-step process in FAA Advisory Circular 150/5325-4B.

Los Angeles World Airports had exactly that work done in 2010, when it needed to know which heavy aircraft could depart from Runway 6R-24L. The consultants pulled takeoff distance at maximum take-off weight, sea level, standard atmosphere, straight from the manufacturers' manuals for every heavy type then using LAX. The results are worth reading slowly.

At maximum weight in standard conditions: the Airbus A380-800 needed about 9,300 feet. The Boeing 757-200, an aeroplane weighing roughly a fifth as much, needed 9,600. The 777-200 needed 9,900, the 747-8 about 10,100, the 777-300 about 10,200, and the 747-400 about 10,500. The longest requirement on the whole list belonged to the 767-200, the A340-300 and the 747-200, at 11,500 feet each.

Read that again. The heaviest airliner ever certified needs less runway than a 747, less than a 777-300, and considerably less than a twin-aisle Boeing that weighs less than a third as much.

Why the A380 beats the 747 off the runway

The answer is not that the A380 has a bigger wing, although it does. It is that takeoff field length is not set by the normal takeoff at all. It is set by the takeoff where an engine quits at the worst possible moment.

Certification requires that the aeroplane can still get safely airborne after losing one engine at decision speed. Lose one of four engines and you lose a quarter of your thrust. Lose one of two and you lose half. A four-engined aircraft is, in that specific and decisive sense, far less damaged by an engine failure than a twin, and the runway numbers show it.

Modern twins compensate with enormous thrust margins — which is why the 777 with GE90s does so well — but the older twins do not. The 767-200 in that LAX table is an early, long-range, relatively low-thrust variant, and it pays for it with 11,500 feet. Engine choice matters as much as airframe: the same table lists different figures for the same type on different powerplants.

What the number actually measures

The figure in those charts is usually the balanced field length, and it is worth understanding because it explains why the numbers are so much longer than the distance you feel yourself using.

During the takeoff roll the crew have a decision speed, V1. Below it, an engine failure means stop. At or above it, it means go. The accelerate-stop distance is the runway needed to reach V1, recognise the failure and brake to a halt. The accelerate-go distance is the runway needed to reach V1, lose an engine and still climb to 35 feet on the remaining engines. Choose V1 so that those two distances are equal, and you have the balanced field length: the shortest runway on which both outcomes are survivable.

So the published number is not how much tarmac a healthy 747 uses. A healthy 747 is airborne long before then. It is how much tarmac a 747 needs available in case the worst happens at the worst moment.

That warning matters for anyone quoting these figures. Airport-planning numbers are deliberately conservative design targets. The number a crew actually works to on the day comes out of the aircraft flight manual and the performance computer, with the real weight, the real wind and the real runway in it.

Then add heat, altitude and water

The design figure is only the start. A runway-length justification prepared for Alexandria International Airport in Louisiana worked the Boeing 747-400 all the way through, and the arithmetic is instructive.

Starting point: a 747-400 with PW4056 engines at its maximum take-off weight of 875,000 pounds, flaps 20, on the airport's mean daily maximum temperature of the hottest month — 92°F. Boeing's chart gives 11,000 feet. Adjust for the airport's 89-foot elevation and you get 11,069. Adjust for the runway's slight gradient and you get 11,111. Then apply the standard wet-pavement factor of 1.15, and the requirement becomes 12,778 feet.

Nearly 1,700 feet of extra runway, bought by rain alone. Take 5 percent off the payload instead and the dry requirement falls to 10,171 feet. This is why heavy aircraft on hot days leave seats and cargo behind: the runway has not changed, so the weight has to.

Altitude does the same thing for a different reason. At 5,000 feet of elevation the air is thin enough that both the wing and the engines underperform, which is why departures out of Denver, Johannesburg and Mexico City are planned around weight limits that would never apply at sea level. The published sea-level number is the best case, not the normal one.

Landing is the easy part

People are often surprised that the answer to “what is the shortest runway a 747 can land on” is so much smaller than the takeoff figure. The same study worked it out: the 747-400 at its maximum design landing weight of 574,000 pounds, flaps 30, needs a FAR landing field length of about 6,234 feet dry. Apply the wet factor and it becomes 7,214 feet.

A 747 lands considerably lighter than it takes off — that is what all the fuel was for — and the certified landing distance already includes a substantial regulatory margin. It is entirely ordinary for a 747 to land somewhere it could not depart from at full weight. The aeroplane simply leaves lighter, or refuels somewhere with more concrete.

The 737 numbers, since everyone asks

Boeing's airport planning data for the 737-700 with CFM56-7B24 engines gives a standard-conditions takeoff length of 5,906 feet at its 154,500-pound maximum weight. Adjust that for a 92°F day at a near-sea-level airport and it becomes 6,930 feet. Apply the wet factor and the planning requirement is 8,008 feet.

Landing is far less demanding again: about 4,700 feet dry at maximum landing weight with flaps 40, or 5,438 feet wet. The older 737-300 lands in roughly 100 feet less, purely because it is certified to land lighter.

So the honest answer to how much runway a 737 needs is somewhere between about 4,700 and about 8,000 feet, and which end of that range applies depends entirely on whether it is taking off or landing, how heavy it is, how hot it is and whether it has been raining.

Which is the real answer to the original question too. A 747 needs roughly two miles of runway at maximum weight on a warm wet day, and rather less than that on a cold dry one at a sensible weight. Anyone who gives you one number without telling you the weight, the temperature and the surface has not answered the question. They have just picked a number.

Sources: Los Angeles World Airports / Ricondo & Associates, “Los Angeles International Airport Takeoff Length Analysis for Runway 6R/24L”, June 2010, compiled from aircraft manufacturers’ airport planning manuals; England Economic and Industrial Development District, Alexandria International Airport Master Plan Appendix B, “Runway Length Analysis”, citing Boeing Airplane Characteristics for Airport Planning for the 747-400, 737-300 and 737-700; FAA Advisory Circular 150/5325-4B, “Runway Length Requirements for Airport Design”; Boeing 747-8 Airplane Characteristics for Airport Planning.