Crosswind on murram: what the demonstrated figure does not tell you.

The demonstrated crosswind component in a typical Cessna 172 POH is 15 knots. The figure on a typical Piper PA-28 is 17 knots. Both numbers were established in flight test, on asphalt runways, in controlled conditions, by manufacturer test pilots. None of those conditions describe a routine training departure from Nakuru, Kericho, or Nanyuki on a hot afternoon into a 12-knot south-westerly across the strip.

Surface type changes the calculus. Murram — Kenya's typical laterite/gravel runway surface across most upcountry training aerodromes — provides lower steering authority on rollout, kicks loose material into propeller and wing-strut paths during taxi, and reacts differently to braking when the surface is wet. The POH's demonstrated figure is a ceiling for asphalt operations. On murram, an operational margin applies before that ceiling is approached.

What the demonstrated figure actually represents

ICAO Annex 6 Part II does not specify how manufacturers must establish the maximum demonstrated crosswind. The figure is a flight-test artefact, not a certified operational limit. For a Cessna 172, the demonstrated component is reached by the test pilot landing the aircraft successfully in a wind whose lateral component, by computation, equals the published figure on a defined asphalt surface.

This means:

• It is not a regulatory limit. Exceeding it does not constitute a violation of certification.
• It is not an operational limit for any student pilot. The student's capability is bounded by their training syllabus and their instructor's judgement.
• It is not a guaranteed achievable figure on every surface. Surfaces with reduced friction coefficient — wet asphalt, packed murram, grass, snow — degrade directional authority during rollout proportionally.

The risk engine in AngaBrief treats the demonstrated figure as the upper bound on asphalt and bands operational risk below it.

The murram differential

Kenyan upcountry training aerodromes — HKNK Nakuru, HKKR Kericho, HKNY Nyeri, HKUK Ukunda Diani, plus many smaller airstrips — operate predominantly on murram. Murram is a compacted laterite surface. Dry murram offers friction close to dry asphalt; wet murram drops to roughly 0.55 of dry-asphalt friction by published surveys, though the figure varies with compaction and aggregate composition.

Three operational consequences:

1. Rollout steering authority is reduced. Differential braking and rudder remain effective, but the lateral force the tyres can sustain without sliding is lower. A side-force that would correct on asphalt may translate into a controlled skid on murram.
2. Take-off rotation behaviour changes. A crosswind from the right that yaws the nose left on rotation has less murram side-grip to work against — the aircraft may weather-vane more aggressively during the few seconds between rotation and positive climb.
3. Propeller and undercarriage abrasion. Aggressive crab angles during taxi pick up gravel into the propeller arc and the wing-strut leading edge. The crosswind figure is unaffected, but the maintenance bill is not.

KCAA does not publish a separate "murram crosswind" limit. The operational margin is at the discretion of the school and the PIC. The typical conservative practice at Kenyan ATOs is to apply an additional 20–25% margin below the demonstrated figure on murram surfaces, and a further 10% reduction when the surface is wet.

How AngaBrief surfaces it

The risk engine's PAVE Environment factor includes two crosswind thresholds:

• +10 points when the computed crosswind component exceeds 80% of the aircraft's demonstrated figure
• +20 points when the computed component exceeds 100% of the demonstrated figure

The 80% threshold is deliberately set below the operational pain point for asphalt operations and at the operational pain point for murram operations. A Cessna 172 with a demonstrated 15-knot crosswind, taking off from murram on a 12-knot lateral wind, will compute 80% of demonstrated — and the risk engine will flag it. The instructor sees the flag in the queue and can make the operational call before the student walks to the aircraft.

The thresholds are school-configurable. A school that operates exclusively on asphalt (HKJK, HKMO, HKNW) may raise its 80% threshold. A school operating primarily on murram (most upcountry ATOs) may lower it to 70%. The configured value is persisted to schools.risk_weights and is rendered alongside the score on the assessment view and the PDF export — an examiner sees both the figure and the school's policy on how that figure is interpreted.

Step 7 of the wizard — Environment — computes the crosswind component from the METAR wind and the runway heading the student selected. The computation lives in the performance module as a pure function and is unit-tested against worked examples. Aircraft demonstrated XW values are seeded per aircraft record in public.aircraft.

What this is not

• Not a hard limit. The demonstrated figure is a flight-test artefact, not a regulatory ceiling. PIC and instructor judgement remain the deciding authority, in accordance with KCAA regulations.
• Not a substitute for the POH. The performance section of the aircraft's POH is the authoritative source for any specific aircraft. Where the POH lists a different demonstrated figure than the seeded record, the POH wins.
• Not a surface-condition oracle. Wet murram, recent maintenance, loose-stone density, and recent rainfall all change rollout dynamics in ways the wizard does not measure. The instructor's pre-flight walk-around remains the source of truth on surface state.
• No automatic surface-condition fetch. Surface state is manual observation by the PIC. A KMD-integrated surface-condition feed is not on the v1 roadmap.

Disclaimer

AngaBrief is a training and decision-support tool. It is not a dispatch authority. Final go/no-go authority rests with the Pilot in Command and the assigned Flight Instructor in accordance with KCAA regulations. AngaBrief does not replace official weather briefings, NOTAM checks, aircraft documentation review, or instructor judgement.

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