Class B In-flight Requirements and Performance Data - ATPL 032 — Performance

The previous lesson supplied the theory; this one earns the licence. Class B chart work is a craft of two motions: forward through the graphs, from the day's conditions to a distance, and backward through the factors, from the runway the aerodrome declares to the distance the graph is allowed to produce. Master the factor arithmetic in both directions and every field-length question in the exam becomes the same three multiplications.

Climb and en-route requirements

The operational rules follow a class B aeroplane past the take-off. The climb requirements demand that the certified climb performance support the departure at the dispatched mass, and the en-route requirement is written around the engine that may stop: a single-engine aeroplane (and a multi that cannot hold height on one engine) must at all points of the route be able to reach a place where a safe forced landing can be made, which is also why commercial single-engine class B work is confined to conditions in which such a landing can be flown: by day, in visual meteorological conditions. It is a route-planning duty as much as a performance one: the line on the chart must never wander further from a landable surface than the glide can bridge.

The single's two calculations

Two worked methods carry the single-engine LOs. The first is obstacle clearance in VMC: from reference zero (the point where the aeroplane reaches 50 ft), the height gained at any distance is the gross gradient times the distance flown, and the question is whether that beats the obstacle's height by the required margin. The second is the net glide after an engine failure: the gross glide gradient (or the lift-to-drag ratio, whose reciprocal it is: an L/D of 10 glides at a 10 per cent gradient) is diminished to a net gradient by the factor the data provide, and the reachable ground distance is the height above terrain divided by that net gradient, stretched by a tailwind and shortened by a headwind.

Working the graphs

The class B take-off and landing graphs are chained carpets, and the method is always the same chase: enter with the temperature, climb to the pressure altitude guide line, run horizontally into the mass panel and follow its guide lines, continue into the wind panel and follow again, and read the distance at the far edge. The same chase run in reverse turns a limiting distance into the field-length-limited mass: enter at the distance the runway permits, chase backwards to the mass panel, and read the heaviest mass the field will accept; the take-off or landing speeds are then read for that mass. The same chart family answers the rest of the block:

Climb data — the rate of climb, the all-engine and critical-engine-out take-off climb figures, and the obstacle-clearance climb data, each entered with pressure altitude, temperature and mass.
Gradients — the still-air gradient for given IAS, altitude, temperature and weight, and the flight-path gradient once the wind component is folded in.
The twin's failure cases — the accelerate-go and accelerate-stop distances, and the take-off flight path with its obstacle-clearance check.
Wind minima — the minimum headwind or maximum tailwind component that makes a marginal take-off legal.
The balked landing — the go-around climb data at the landing mass and configuration.

The dispatch factors, forwards and backwards

Raw graph output is not yet a legal take-off. For commercial air transport, the corrected take-off distance must fit 1.25 times over into the take-off run available when no stopway or clearway is declared; where they are declared, the corrected distance must fit within TORA unfactored, within TODA times 1.15, and within ASDA times 1.30. Before the 1.25, the surface and slope corrections apply: dry grass stretches the graph distance by a factor of 1.2, wet grass by 1.3, and every 1 per cent of upslope adds 5 per cent to the take-off distance. The landing mirror is the 70 per cent rule: the corrected landing distance may use no more than 70 per cent of the landing distance available, which is the same statement as multiplying the landing distance by 1.43; grass or a wet surface each add a factor of 1.15, and each 1 per cent of downslope adds 5 per cent.

The backward run is the exam's favourite: given TORA, TODA and ASDA with a slope and a surface, divide each declared distance by its factor chain, take the most restrictive result, and that defactored distance is what the take-off graph may legally deliver; enter the graph backwards with it and the field-length-limited mass appears. The landing version divides the LDA by 1.43 and the surface factors to find the defactored landing distance. And the same chains run forward answer the minimum-runway question: a defactored distance multiplied back through its factors gives the minimum TORA, TODA or landing distance that must be available.

Describe the class B climb and en-route operational requirements

Calculate single-engine obstacle clearance from gross gradient, obstacle height and distance from reference zero, and the net glide gradient and distance

Determine distances, climb data, accelerate-go/stop data, the take-off flight path and obstacle clearance, minimum headwind or maximum tailwind, and field-length-limited masses and speeds from the graphs

Calculate defactored take-off and landing distances from declared distances, slope and surface, and the minimum TORA, TODA or landing distance required

Climb and en-route requirements

The single's two calculations

Check you understood

Working the graphs

The dispatch factors, forwards and backwards