Windswept

During a weather briefing, I noted strong winds from the southwest forecast to be a blistering 40 knots at 3,000 feet msl. Based on that, we should have been able to make it to Smyrna Airport with almost twice the required night VFR reserve left upon landing.

We left Bowman Field in darkness. Sufficient ground lights and visibility existed for a good VFR night flight. We had to fly east of Fort Knox, and the best way to do it in VFR was to follow Interstate 65 while verifying the position with the nearby VORs. This was the pre-GPS era, and the smart way to know groundspeed was simply to ask ATC. Already on climbout I noticed that we were barely moving over the ground. Flying southbound and following I-65, we saw cars speeding below us as if we were hovering. We were now cruising at 2,500 feet msl and 100 knots true airspeed. I abandoned the idea of climbing higher to obtain better radar coverage. “Say groundspeed,” I said to Louisville Departure. ATC responded: “Showing 30 knots.”

No wonder these cars below were overtaking us. The actual headwind was about 70 knots, and when we got down to 2,000 feet msl, the headwind component was still around 50 to 60 knots.

That 20-year-old Cessna 152 had a no-wind range of more than 300 nautical miles and endurance of three hours (and a few minutes) with 45 minutes reserve. Usable fuel was about 24 gallons.

We had to land to refuel—and made it to Bowling Green Airport (BWG), still 60 nautical miles short of our destination, with only three to four gallons (practically 30 flight minutes) left in the tanks. Bowling Green is about 100 nautical miles from Bowman Field including a deviation around the Fort Knox restricted area. It took us more than three hours just to reach Bowling Green, at an average groundspeed of less than 35 knots and flying very low. There was no way we would have reached any of the Nashville-area airports or our destination of Smyrna Airport.

My passenger literally kissed the asphalt on the FBO ramp upon landing in Bowling Green. I was tempted to do the same. If headwinds had been just five knots stronger, we wouldn’t have made Bowling Green.

So what is the lesson? Wind affects flying aircraft in many ways, and mostly adversely in cruise. Say a pilot flies at 100 knots true airspeed between points A and B, which are 100 nautical miles apart. In a no-wind situation it would take one hour from A to B and one hour from B to A for a total of two hours.

The same round-robin flight in 20 knots headwind nets a longer round trip. That would imply a groundspeed of 80 knots from A to B and a return groundspeed of 120 knots. So what we lose from A to B we will simply regain when B to A—right?

Time A to B is now 100 nautical miles divided by 80 knots, or an hour and 15 minutes. Time from B to A is 100 nautical miles divided by 120 knots, or 50 minutes. The total is 2.083 hours, or two hours and five minutes.

Does a pure crosswind affect cruise flight while staying on a given course? There are no headwind or tailwind components, so the groundspeed should not decrease, right? Not so fast. A wind correction angle (WCA) must be assumed into the wind to offset the crosswind component, and that will reduce groundspeed. The stronger the crosswind, the lower the groundspeed.

Flying in a 100-KTAS airplane with a direct crosswind of 40 knots will require a wind correction angle of almost 24 degrees, and the resulting groundspeed will be less than 92 knots. In the extreme case of 100-knot crosswind, an angle of 90 degrees will be required, resulting in zero groundspeed in either direction.

The fact is that any steady wind will always affect aircraft adversely on round-robin flights. A steady headwind is good for landings, takeoffs, and clearing obstacles during climb, but in cruise it really hurts. That headwind does more harm than the corresponding tailwind on the return leg does good.

Not only does headwind reduce the groundspeed of an aircraft in cruise, but it also prolongs the exposure to it. But won’t tailwinds always increase groundspeed? Unfortunately, no. It depends on the magnitude and the angle. As the wind speed increases, there is an increasing critical tailwind angle at which the direct tailwind component will just offset the lost groundspeed because of the required wind correction angle to maintain course. At that angle, groundspeed equals true airspeed.

Imagine an airplane flying 100 knots true airspeed in a steady relative 110-degree tailwind of 70 knots. The tailwind component is 24 knots. But the crosswind component is almost 66 knots, requiring a drift angle of 41 degrees and reducing the groundspeed. The final groundspeed is actually 99 knots, less than true airspeed.

An airplane on a fixed course flying 100 knots true airspeed with 45-degree tailwind (135 degrees from the nose) at a steady 141 knots will turn perpendicular to the desired course. The groundspeed equals true airspeed because of the 100-knot tailwind component. An observer on the ground will see the airplane moving sideways, but for the pilot there will be nothing unusual. The airplane creates its own relative wind. Any tailwind of this magnitude at angles less than 135 degrees will actually cause groundspeed to be less than true airspeed. But no sane pilot would fly a 100-knot airplane in a 141-knot headwind.

The critical parameter is the ratio of the wind speed to the airplane’s true airspeed (WS/TAS). One should generally not be flying if this ratio exceeds 0.25 for student pilots to 0.4 for experienced, expert pilots. For a Boeing 767-300ER flying across the Atlantic Ocean, this ratio only occasionally could reach 0.3 to 0.4 in strong jet-stream winds. The supersonic Concorde was much less affected by headwinds. Consider that actual winds may be significantly stronger than those predicted in a forecast. When I was flying in that Cessna 152 many years ago, I almost ran out of options as the local winds were 20 to 30 knots stronger than already forecast strong winds for terminal areas. In my case the (WS/TAS) factor was steadily between 0.6 and 0.7.

For student pilots doing cross-country flights, I would suggest not flying if headwinds are forecast to exceed 25 to 30 percent of the aircraft’s cruise speed at a given altitude. The crosswind component should not exceed 40 percent of cruise speed; otherwise, an extreme wind correction angle is required, with significant groundspeed loss. This crosswind also makes things difficult with fewer options if an alternate airport is needed. If actual winds are significantly stronger than forecasted (more than 30 to 50 percent), it is perhaps better to look for an alternate landing airport promptly if an altitude change does not bring any relief. The faster the aircraft, the less susceptible it is to adverse wind effects in cruise. However, most GA training aircraft cruise around 100 knots and are thus occasionally vulnerable to terrestrial wind effects.

Author Nihad Daidzic is president of AAR Aerospace Consulting LLC, a professor of aviation at Minnesota State University, Mankato, an ATP, and a CFII/MEI/glider instructor.