On a recent trip out of Washington Dulles to San Francisco, our B777 met with 200+ mph headwinds (as per the passenger entertainment system) at 35,000 ft for the first half of the route, before enjoying 50 mph tailwinds at 38,000 ft over Utah, Nevada and finally California.

The ground speed was displayed at 400 mph for the first half of the trip, then closer to 550 mph. Does that mean that air was rammed into the engines first at 400+200 = 600 mph, then in the second half of the trip at only 550 - 50 = 500 mph?

How do the engines cope with various intake airflow speeds?

  • $\begingroup$ These are two good questions, however the format of this site works best when only one question is asked at a time. I suggest you remove the second question from here and ask it again separately. Not only will you get a better answer, but it will be easier for future visitors to find their specific question. $\endgroup$ – Ben Jan 4 at 8:53
  • $\begingroup$ @Ben - you are right. Let me work on that. $\endgroup$ – Frank Jan 4 at 15:22
  • $\begingroup$ While the change you point out is not sensed by the engine, there is a more significant change actually sensed by the engine between takeoff (full thrust required at low airspeed), cruise (reduced thrust at maximum airspeed), end of descent (idle at reduced airspeed). $\endgroup$ – mins Jan 4 at 21:47

The airplane isn't aware of the winds. It just moves through the air mass at a certain speed, and if the air mass is moving it adds or subtracts from the aircraft's speed over the earth.

Drive your boat along a river with a 10 mph current upstream, then downstream, both at 25 mph on the boat's speedo. The whole time your boat's showing 25 and the hydrodynamic forces acting on the hull are 25 mph forces. What's your speed relative to the shoreline going upstream compared to going downstream? 15 going upstream and 35 going downstream right?

Exact same thing with airplanes. Navigating ships allowing for ocean currents is exactly the same as navigating airplanes in "air currents", they have "tailwind" currents and "headwind" currents and cross currents where you have to aim for point X to actually proceed to point Y, except things are happening in slow motion in the ship.

  • $\begingroup$ John - the question is more specifically about how/how much a jet engine can deal with varying intake airflow speeds. There is also a separate question about how the wind factors into trip planning. $\endgroup$ – Frank Jan 4 at 2:32
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    $\begingroup$ I think John answered the first question very well... the question was specifically, "does a 200mph headwind and a 400mph GS mean that air was rammed into the engines at 600mph?" Explaining the differences between GS and Air Speed, using a river analogy, is perfectly on-point. There are still 2 unanswered questions: how do engines deal with varying intake airflow speeds (kind of nullified because the premise of the question is that there were varying intake speeds, which there weren't); and, do airliners adjust their AP to delay potentially early arrivals to avoid disrupting airport ops. $\endgroup$ – Jimmy Jan 4 at 3:51
  • $\begingroup$ @John K - I move the second part of the question, about arrival times, to a separate question, as per Ben's comment. $\endgroup$ – Frank Jan 4 at 16:10
  • $\begingroup$ @Jimmy - nullified? I thought that depending on the wind conditions, for a given ground speed, the air would flow into the engines at different speeds. Is it not the case? I was thinking that when designing jet engines, you would have to allow a range of intake speeds. $\endgroup$ – Frank Jan 4 at 20:19
  • $\begingroup$ No the intake speed is the airplane's airspeed so it has to cope with the airplane's speed range, not wind effects. Same as the water intake speed of a jet boat's impeller is the speed of the boat on the water, not the speed of the boat plus or minus the speed of the current. The only time the wind effects engines is on some engines there are special procedures or setting thrust when on the ground in a crosswind or tailwind. That's on the ground, where the air is coming from the side or behind. $\endgroup$ – John K Jan 4 at 20:42

At an altitude of 35000 feet, 600 mph works out to about Mach 0.904. (There's a handy calculator for this here: https://www.grc.nasa.gov/www/k-12/rocket/machu.html.)

A Boeing 777 can fly at least as fast as Mach 0.89 at cruising altitudes. (Source: https://www.flightglobal.com/news/articles/flight-test-boeing-777-300er-fast-and-heavy-176585/.) It's possible you actually were flying at Mach 0.89 and the "600 mph" includes some rounding up.

Of course the aircraft is designed to fly slower as well. Otherwise you couldn't take off or land.

So yes, the engines are designed to deal with getting a lot of air in their intakes, up to some maximum amount, and also to operate while getting a lot less air. When you drive a conventional automobile it also is able to operate on much less air than the maximum it can take.

The Mach number on the second part of the trip was only about Mach 0.76 according to your numbers. But with a tailwind, you can get more efficient flight (fewer pounds of fuel per mile) by flying slower and letting the tailwind carry you forward. Going into a stiff headwind you want to fly much faster to reduce the amount of time you're fighting that headwind. (This is oversimplified, because the altitude also affects the optimal cruise speed, but the optimal cruising speeds tend to be greater in a headwind than in a tailwind.)


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