AEROSPACE: Proper preflight planning prevents...

Balanced Field Length

Hat Tip to NeptunusLex

December 18th, 2008 · Flying
The pilots of single-engine aircraft are required to compute expected take-off distance using aircraft gross weight, temperature, pressure altitude, forecast winds and runway gradient, taking into account obstacle clearance at the departure end. It’s good to know that the runway you intend to use is long enough for the use you intend.

In the A-4 Skyhawk, students were additionally expected to calculate a “line speed” check to ensure that the engine was operating at the minimum acceptable thrust after having traveled a thousand feet down the runway. Because if you lose your only engine at any speed prior to rotation, getting the aircraft airborne for any length of appreciable time becomes very difficult, and knowing that the engine is in fact operating within tolerances early in the take-off roll avoids all of the high-speed drama attendant to max braking on a full fuel load with the boundary fence looming ever larger in your field of view. Departure end arresting gear gives a pilot other options in the short hairs, but the availability of such equipment forms no part of the calculation - tailhooks have been known to skip wires.

In multi-engine aircraft, things can get a little more complicated. Based on the same factors above plus runway length, there can come a time when a critical emergency on the roll forces a pilot to decide whether to reject the take-off or continue with the remaining engine. At the computed airspeed of V1(decision speed), the pilot has two options available to him should an engine fail: 1) Reject the take-off, and stop the aircraft in the distance remaining, or 2) accelerate towards V2, take-off speed.

On runways of 8000 feet or less on a hot day, the F-5E had a narrow speed band between Vno-go and V1 that we referred to jokingly as Vscrewed, meaning that insufficient runway remained either to abort or continue for take-off. In that case, the drag chute and the tailhook were the only options remaining (apart from the ejection seat) and the hook itself a mere, slender reed of a thing, placed there mainly to assuage the wounded egos of tailhooker wannabes.

The bias inn most multi-engine aircraft I flew after reaching V1 was to continue the take-off and deal with whatever emergency you had airborne, so long as the aircraft was flyable. Airplanes are made to fly and basically want to, but making them stop on a rapidly diminishing runway at NASCAR speeds and max gross weight requires more than the usual amount of finesse and associated stress.

The concept of V1 has an associated metric known as “Balanced Field Length”, which is the minimum length of runway required for an aircraft to accelerate towards V1, lose the critical engine and either abort or continue to the take-off, meeting all applicable take-off performance criteria such as obstacle clearance and rate-of-climb.