A comment by a fellow pilot got me thinking about headwinds and tailwinds. I started flying with serious misconceptions about how a headwind or tailwind affects a flight, and some of the bogus rules of thumb only makes things worse. I’m going to take a quick look at three popular misconceptions here — that you make up for a headwind on the return trip, that your average groundspeed over many trips is a good indication of your plane’s true airspeed, and that you should fly faster into a headwind to save fuel.
Making up for a headwind
First, the easiest one. Let’s say that you’re flying on a round trip in a single day, and the wind is forecast to be the same all day. On your way outbound, you will have a headwind, and on your way back home, you will have a tailwind. Sounds about even, right? To try it out, consider a plane with a 120 kt true airspeed (like a Warrior or a Cessna 172) flying 300 nm each way:
In other words, you never make the time up, because you spend more of the flight in the headwind than the tailwind, by definition. There are tricks, of course, like flying low westbound to get a weaker headwind and flying high eastbound to get a stronger tailwind, but averaged over many flights, the best wind is still no wind at all.
Average ground speed
And that comes to the second point. When people want to challenge the true airspeed figures put out by the airplane manufacturers, they often pull out their average groundspeeds, which are inevitably 10 knots slower or more, for which they usually blame the manufacturer’s marketing department. Part of the difference can be explained away by density altitude (nobody always flies at 7,000-8,000 feet density altitude), low power settings (many pilots are shy about setting 75% power), or climb, approach, etc., when the plane is flying outside its optimum conditions, but another big part of the difference is the wind. For example, in the table above, the plane’s average groundspeed would be 120 kt with no wind, 117 kt with a 20 kt wind, or 107 kt with a 40 kt wind. Average groundspeed is a more accurate indication of how long trips will take in a plane, but it is a different measurement than the plane’s true airspeed, because any wind at all hurts the average, and there’s almost always some wind.
Fly faster into a headwind
Flying faster into a headwind will definitely get you home sooner, but it won’t usually save gas, despite what many pilots, including flight instructors and textbook writers, try to tell you. To demonstrate, I’ll use another table. At 8,000 ft density altitude, a Cessna 172p will fly 121 ktas burning 8.6 gph at 75% power, 112 ktas burning 7.4 gph at 65% power, and 100 ktas burning 6.2 gph at 55% power according to its POH; this table shows the time and fuel to fly 300 nm with different headwinds:
|Headwind||55% power||65% power||75% power|
|Time||Fuel (gal)||Time||Fuel (gal)||Time||Fuel (gal)|
Obviously, if you just want to get home, you are better off speeding up and paying for the small amount of extra gas–you’d have to be pretty dedicated to fuel savings to make your trip an hour longer to save 1.5 gallons of fuel. However, if you’re worried about running dry and there’s nowhere to turn around to (let’s say that you’re halfway between Greenland and Iceland), speeding up is not necessarily the best choice. Even at 55% power, you’re still burning half a gallon less gas than at 65% power and almost two gallons less than at 75% power flying directly into a 30 kt headwind. With a 40 kt headwind, speeding up to 65% power starts to make sense, but 75% will still burn a full gallon more fuel; in fact, you need to get up to a 60 kt direct headwind before you will save fuel by speeding up to 75% power.
Note that these numbers are for a very slow plane. If you’re flying a fast single, like a Lancair or Cirrus, or a twin, you will probably always be considerably better off at 55% for conserving fuel, unless you’re flying into a hurricane. I imagine that cross-ocean ferry pilots pretty-much always fly at low power settings, no matter what the wind is like.
Before I crossed the “pond”, I also presumed that efficiency would be a top concern. However, the experienced ferry pilot I had with me practiced maximum speed upon “feet wet”, to get it over with. Sitting up there, flying along slowly and efficiently is more time for the weather to go bad, stuff to break, etc. You don’t want to be anywhere close to your fuel limits over the Atlantic, have a ferry tank installed if there is the slightest doubt about range.
Aside from that, I think the points about fuel efficiency are very practical for normal US flying.