Equivalent Still Air Distance - ESAD

Presented by Marc Brodbeck

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ESAD

ESAD  (Equivalent Still Air Distance) nautical air distance flown, accounting for headwinds or tailwinds

Flying into a headwind will increase the ESAD compared to zero winds or a tailwinds

Formula for ESAD (nautical miles):ESAD = (TAS * Ground mileage) ÷ (TAS + WV)Where;TAS = True Airspeed (cruise speed in knots)WV = Wind velocity Example:TAS = 440 knotsWV = -80 knotsGround Mileage = 3600 nautical miles

ESAD = (440 * 3600) ÷ (440 – 80)ESAD =4400nm

ESAD distance (longer due negative effects of headwind)

Ground mileage over the surface

Headwind

ESAD distance (shorter due positive effects of tailwind)

Ground mileage over the surface

Tailwind

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ESAD example for eastbound trip

Air Miles or Nautical Air Miles (NAM) is also another common term which is synonymous with ESAD

Example Flt Plan KBDL-EDDH, A330-342 Flight Plan Distance = 3318nm

(3% for normal departure, airways, and arrival procedures)

Avg. Wind Component = 49kt Tailwind

ESAD (Air Distance) = 2998nm

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ESAD example for westbound trip

Air Miles or Nautical Air Miles (NAM) is also another common term which is synonymous with ESAD

Example Flt Plan EDDH-KBDL, A330-342 Flight Plan Distance = 3427nm

(6% for departure, airways, and arrival procedures)

Avg. Wind Component = 24kt headwind

ESAD (Air Distance) = 3611nm

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Example: EHAM-KBDL (Amsterdam to Hartford)

The annual average headwind for the direct route is a 40 knot headwind (- = headwind)

Example:TAS = 468 knotsWV = -40 knotsGround Mileage = 3071 nautical milesESAD = (468 * 3071) ÷ (468 – 40)ESAD =3358 nm

The annual average tailwind for the returns route is a 37 knot tailwind (+ = tailwind)

Example:TAS = 468 knotsWV = +37 knotsGround Mileage = 3071 nautical milesESAD = (468 * 3071) ÷ (468 + 37)ESAD =2846 nm

The ESAD varies by ~500 nm depending on the direction of flight, due to winds

Longer Route to avoid winds

Direct routeWind

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Usefulness ESAD can be used to normalize for aircraft payload/range capability Long haul routes with strong winds will have large ESAD variability over the

course of 1yr ESAD range is a primary factor in judging aircraft performance Manufacturers (OEMs) typically quote still-air range, ESAD normalizes for

wind impact

Data ESAD data normally comes from upper air data wind interpolations Historical ESAD data can be derived from Boeing PC Windtemp or climate

archive/research data, e.g. NCDC Current/forecast ESAD can be computed manually; using brute force

methods via upper air wind data

Practical ESAD