Class B · runway, wind and technique

Why does the same aircraft need a different runway today?

Ground roll runs from the start to lift-off; take-off distance includes the airborne portion to the specified screen height. Maximum allowed take-off mass is the lowest applicable structural and performance limit. Landing distance includes the airborne part from the landing screen height and the ground roll to a stop; maximum allowed landing mass must also meet climb requirements.

Build the take-off distance

Ground roll runs from the start to lift-off; take-off distance includes the airborne portion to the specified screen height. Maximum allowed take-off mass is the lowest applicable structural and performance limit. Landing distance includes the airborne part from the landing screen height and the ground roll to a stop; maximum allowed landing mass must also meet climb requirements.

More flap usually lowers lift-off speed and shortens ground roll, but adds drag and can worsen climb. Uphill slope, soft ground, contamination and high density altitude usually increase take-off distance. Fixed-pitch propellers operate away from their best blade angle over part of the acceleration; constant-speed systems change blade angle to maintain selected RPM. Neither creates constant static thrust at all speeds. A rolling start before take-off power is established can consume extra distance; use data matching the actual technique.

Resolve the wind before using it

With angle Δ between runway heading and wind-from direction, headwind = wind speed × cos Δ and crosswind magnitude = wind speed × |sin Δ|. A negative headwind is a tailwind. Use the same true or magnetic reference for both directions. In normal Part-CAT class-B calculations credit no more than 50% of reported headwind and account for no less than 150% of tailwind, unless the applicable approved data already incorporate the rule.

Crosswind restrictions protect directional control and depend on the surface and approved aircraft/operator limits. A demonstrated crosswind value is not automatically an AFM limitation: read its status. Hot/high conditions raise TAS at the same IAS and may reduce power, worsening both take-off and go-around. Wetness can greatly increase landing roll even where the take-off ground run changes only modestly.

Keep the airborne path honest

Arriving fast or above the 50-ft screen extends the airborne distance and increases touchdown energy. A 10% groundspeed increase gives about 21% more kinetic energy at the same mass; it does not imply a universal 21% total-landing-distance penalty. For class-B multi-engine CAT obstacle assessment, the take-off path begins at 50 ft at the end of the factored take-off distance and extends to 1,500 ft above the take-off surface.

The normal class-B obstacle domain uses 90 m + 0.125D, with half-span + 60 m + 0.125D permitted for span below 60 m, with the applicable caps and conditions. Read whether a turn, navigation accuracy or visual reference changes the accountability width. Vertical clearance and lateral accountability are separate checks.

How do you turn a climb or glide chart into terrain clearance?

For a multi-engine class-B CAT take-off path, assume critical-engine failure at the point where visual reference for obstacle avoidance is expected to be lost. From 50 ft to that assumed failure height use the average all-engine climb/transition gradient multiplied by 0.77; thereafter use the AFM OEI en-route gradient. The required vertical obstacle clearance is 50 ft. The normal rule permits no bank before 50 ft and at most 15° thereafter. VFR-by-day exceptions apply to specified elements of CAT.POL.A.310; 300 ft is a common guidance case, not a universal failure height.

Class B has different safeguards

For a multi-engine class-B CAT take-off path, assume critical-engine failure at the point where visual reference for obstacle avoidance is expected to be lost. From 50 ft to that assumed failure height use the average all-engine climb/transition gradient multiplied by 0.77; thereafter use the AFM OEI en-route gradient. The required vertical obstacle clearance is 50 ft. The normal rule permits no bank before 50 ft and at most 15° thereafter. VFR-by-day exceptions apply to specified elements of CAT.POL.A.310; 300 ft is a common guidance case, not a universal failure height.

For en-route multi-engine class B, the en-route gradient is the gross climb gradient reduced by 0.5 percentage points, or the gross descent gradient increased by 0.5 points; continuation must meet minimum safe altitudes to 1,000 ft above a suitable aerodrome. Single-engine operations need route conditions allowing a safe forced landing after power loss, subject to specific approvals and exceptions such as SET-IMC. These rules cannot be replaced by a positive all-engine climb figure.

Compute a path in one frame

To calculate obstacle clearance, begin at the defined screen/reference point, apply the required gradient to the horizontal distance beyond that point, and compare aircraft height with obstacle height above the same datum. If the chart gives ROC, convert with TAS for an air gradient or with GS for the ground path. Wind cannot be omitted from a terrain calculation.

For single-engine net glide exercises, the class-B rule increases the assumed descent gradient by 0.5 percentage points. Convert L/D to a gradient first: L/D 10 gives a 10% gross descent gradient. Adding 0.5 points gives 10.5%, not 15%. Apply the wind correction in the frame prescribed by the question. Deduct terrain elevation from altitude before converting height to distance.

Can you make the performance decision?

Four original questions. Work from the stated conditions and review the reasoning.