The impact of wind on real world aircraft performance

Alasdair Whyte
By Alasdair Whyte March 1, 2012 16:14
David Wyndham, owner of Conklin & de Decker, looks at how wind can affect an aircraft’s performance.

Aircraft performance, especially range, is like gas mileage in a car. Much of the performance data points shown are maximums. Given that the assumptions are the same, they are very useful in comparing aircraft. Be wary of translating the maximum to the “real world.” This is especially true of range. Too many buyers get an aircraft that they believe can do the trip nonstop only to discover that “nonstop” has restrictions. They get quite upset when they end up with a fuel stop en route.

The first thing that impacts the real world range is winds. In the Northern Hemisphere, prevailing headwinds run on a West-to-East pattern. So trips from Europe to the US have headwinds while trips from California to New York have tailwinds. Those winds vary seasonally and by altitude.

Boeing publishes wind probability data for many of the common air routes in the US and worldwide. A common wind data point is the 85% Probable Wind. That means that the wind on that route will be no worse than that value 85% of the time. Here in the US, flying from the East Coast to the West Coast can have an 85% probable headwind of around 70 knots at 39,000 feet. So flying east to west, you should have 70 knots or less headwind 85% of the time for example.

When looking for that nonstop airplane, you need to factor in those probable winds. If the route is 2,100 Nautical Miles (NM), that is across the ground. Headwinds effectively increase that required distance. If the aircraft cruises at 430 knots in a 70 knot headwind, its ground speed will only be 360 knots. Fly into this headwind for five hours and your trip has effectively increased by 350 miles – almost an hours’ flight time. Looking at this another way, to fly that 2,100 NM trip in a 70 knot headwind requires an aircraft with about 2,450 NM range (with no wind).

Other things that reduce the fuel efficiency and thus maximum range of the aircraft:

  • Payload – heavier aircraft burn more fuel at a given speed and may require a lower initial cruise altitude until they burn off enough fuel to reach a higher, more fuel-efficient altitude.
  • Temperature – on very warm days the aircraft may take longer to climb to altitude, or even require a lower initial altitude.
  • Circuitous air routes – while airways routes typically add no more than about 3% to the straight-line distance, some routes may add more due to airspace restrictions or transoceanic routings.
  • Long, over-water trips may require alternate airports that are a significant distance away from your destination. This will reduce the available fuel load for the trip.
  • Poor weather over a large area may mean a circuitous route and may also require an alternate airport a significant distance away.

Factor in headwinds, heavy passenger loads, and a warm day and that 2,100 NM trip may not be non-stop anymore. So if you are looking for 2,100 NM nonstop trip with high-probability, you may be looking for an aircraft that has 2,600 NM range with your anticipated payload.

When evaluating aircraft, the maximum ranges, maximum speeds, payload capability, etc. can all be important considerations. But when you are looking at specific trips, you need to factor in some real world considerations appropriate for your trip conditions.

Alasdair Whyte
By Alasdair Whyte March 1, 2012 16:14

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