The aircraft OEMs and sellers always seem to haver a war of words going on over “maximums.” Who has the fasted aircraft, the most range, the biggest cabin, the most headroom, etc? When evaluating aircraft, you need to take the maximums into account, but critical is understanding the real world limits on those maximums.
Aircraft performance maximums can be very useful in comparing aircraft. Especially if you know the conditions they were assuming for the calculation. But 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.
As pilots, we know and understand these restrictions, but many of the folks in the back fail to understand these, until after the sale. Better to educate them upfront than be accused of backpedaling after the fact.
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.
Last winter on an airline trip, we hit 135 knots on the nose for much of the Baltimore to Phoenix trip. Not sure, but think that was a 99.9% “Probable” wind leg. No aircraft can do the trip 100% of the time, but make sure the maximum range is suitable for your typical maximum trips. Here is a conservative shortcut that gets the job done.
When looking for that non-stop airplane, you factor in those probable winds as a reduction in cruise speed. 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. How many bags does the boss bring?
- Temperature - on very warm days the aircraft may take longer to climb to altitude, or even require a lower initial altitude. Temperature may also effect engine-out departure restrictions.
- 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. In this case, a 2,600 NM maximum range is a valid requirement. 3,000 NM range is nice, but 2,600 NM will do the trip 99% of the time.
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.