En-route performance

032 PerformanceChapter 4

Range and endurance

Formula — Specific range

SR = nm ÷ kg = TAS ÷ Fuel flow = TAS ÷ (SFC × PR)

Jets: best SR at 1.32 VMD. Props: best SR at VMD.

Key fact

Range — distance with available fuel; efficiency measure. Endurance — time aloft; found when fuel consumption is least. Best endurance speed is less than best SR speed. Best endurance: jets VMD, props VMP.

Key fact

Tailwind increases range but has no effect on endurance. For max ground range, fly slower in tailwind, faster in headwind. Temperature ↑ and weight ↑ both reduce range and endurance (SFC ∝ mass).

Cruise climb profile

Key fact

Climbing through low altitudes, IAS constant (VMO limiting). Through higher altitudes, Mach constant (MMO limiting). Long Range Cruise ≈ 4% faster than max range — closer to max SR.

Cost index

Formula — Cost index

Cost Index = Cost of time ÷ Cost of fuel

Low CI → low speed / max range. High CI → high speed / shortest time.

Drift-down (OEI en-route)

Key fact

When remaining thrust is less than drag, aircraft cannot maintain speed/altitude — drift-down to an altitude that can be maintained. Requirements: 1000 ft clearance at level-off; 2000 ft during drift-down (Class A vertical interval); positive gradient at 1500 ft AAL. If not met, jettison fuel ASAP.

Key fact

Best drift-down speed (jets): VMD = VX (least rate of descent). Props/turboprops: VMD = VY (best OEI rate of climb speed). Net gradient = gross + 0.5% (Class B drift-down).

ETOPS

Key fact

Non-ETOPS: 60 minutes at OEI cruise speed in still air, ISA. ETOPS approval can extend this. Greatest clearance — fly VMD (VX jets, VY props). Greatest range — 1.32 VMD (jets), VMD (props).

Buffet and manoeuvring ceiling

Key fact

Thrust available vs buffet onset (low or high-speed stall). VS = VDD at aerodynamic ceiling. 0.3G margin (40° AoB) applied for maneuvering ceiling. Optimum altitude may not have optimum winds — trade applied when tailwind exceeds losses.

CAP graph calculations

Key fact

CAP gives gross performance; aerodrome factors → gross specific; regulatory factors → net. Climb graph gives air distance — use NAM ÷ TAS = NGM ÷ GS. At reference zero, aircraft already at 50 ft — subtract/add 50 ft. Wind: Wind speed × cos(difference in angles) instead of wind graphs. Calculate pressure altitude if QNH ≠ 1013. Do not confuse TAS and IAS.

Braking and landing performance planning

Key fact

Landing considered in still air and forecast winds — most restrictive gives FLLLM. Rule: use 0 wind or the tailwind. Multiple runways: least of highest still-air and wind-effective masses. Brake cooling: within limits — no wait; outside limits — wait 53 minutes and check thermal plugs (CAP MRJT 5.2).

Exam trap

Fuel mileage (NM/kg) lowers with forward CG. Reciprocating engine at constant AoA, mass and configuration: drag unchanged with altitude but TAS increases.