By Robert N. Rossier, EAA 472091
This piece originally ran in Robert’s Stick and Rudder column in the October 2020 issue of EAA Sport Aviation magazine.
Recent world events have reminded us all to have a “go the distance” mindset in our lives — and that’s something that we take into the cockpit with us as well. Situations can develop or change more quickly than we might anticipate. Our ability to extract the most performance from our aircraft is often the leverage we need to ensure a successful outcome. Whether we’re trying to extend our range, maximize our endurance, or glide to a safe landing, knowing what to look for and what to do puts us in better control of the situation.
Some in-flight scenarios might require us to go the greatest distance we can with the fuel available. We have three tools to work in this scenario: our knowledge of power settings, density altitude, and the prevailing winds. Our pilot’s operating handbook typically provides us with maximum range data for zero-wind conditions at various power settings. Using a Cherokee 180 F model as an example, we find that at a cruise altitude of 3,000 feet, if we reduce our power from 75 percent to 55 percent, our range increases from 700 statute miles to just over 800 miles. That additional 100 miles could be critical, depending on the situation we’re in, although it will take a little more time to get there at the airspeed delivered by the lower power setting. Even if we’ve already used half our fuel, being able to travel an extra 50 miles might give us an out we didn’t know we had.
Altitude also affects our range. With an increase in altitude, our true airspeed increases due to density altitude effects, and that translates to more ground covered. In the same zero-wind example, if we were to climb from 3,000 feet to 10,000 feet, we could squeeze another roughly 40 miles of range out of the old Cherokee.
The zero-wind assumption is seldom the reality we face. Generally, winds increase with altitude, so we need to know which way the wind is blowing. The effect of climbing into a greater headwind could easily negate the benefits of improved true airspeed. If the wind is behind us, we do even better.
When trying to gain distance into the wind, we do better with a slightly higher power setting and lower altitude rather than using the rock-bottom 55 percent power setting and climbing higher. On the other hand, if the wind is at our back, we should climb as high as we can and reduce power as much as possible to get the greatest range. To get a good idea of the combined effects, we can work a few scenarios for the aircraft we frequently fly, using realistic winds aloft examples, to see what combinations offer the best results.
Other scenarios might put us in a position where we simply want to stay aloft as long as possible. Perhaps we’re at our destination waiting for the weather to improve or a runway to be cleared. In this case, what we’re looking for is best endurance. Generally speaking, our best performance is going to be at a low (55 percent) power setting and at a lower altitude. The airspeed we want will be what’s called minimum sink speed. It is usually not published but is a little slower than best glide speed. Using the performance charts from a Cessna 152 (1979 model in this case), we find that our endurance with 24.5 gallons of fuel improves from a little more than three hours to better than four hours by reducing power from 75 percent to 55 percent. We can gain yet another hour of endurance by reducing power to 45 percent.
Remember, we’re not trying to get anywhere, we’re just trying to stay airborne, so the lowest power setting at which we can stay aloft is the best we can do. The charts also show that endurance is slightly better at lower altitudes than at higher altitudes. We can look at the charts for the aircraft we usually fly to get a better sense of the endurance at various power settings.
Getting the Best Glide
One common emergency we practice is the power-out scenario, where we glide to a runway or off-field location for an emergency landing. We know that the key to glide performance is proper speed control, and we thus commit our best glide speed to memory. For most light aircraft, the best glide speed is roughly halfway between the published speeds for best angle (VX) and best rate (VY).
Although we might like to focus on that single, published best glide speed, we do well to recognize that the speed and performance vary with aircraft weight. As weight increases, so does our best glide speed. If the aircraft is below gross weight, best glide will typically be slightly lower than published. The difference may only be a couple knots or so, but it’s worth noting. While the airspeed we use for best glide doesn’t change with density altitude, the glide distance does. Since at higher density altitude we’re traveling at a higher true airspeed, we actually can glide a bit further. And the more altitude we have to start with, the greater the difference in glide distance.
Getting the best glide performance from our aircraft is not just a matter of maintaining the best glide speed. We also need to clean up the aircraft aerodynamically to reduce drag. That means getting the flaps and landing gear retracted for starters. Retractable landing lights should be retracted in daytime and cowl flaps closed to reduce drag. For aircraft with a constant-speed prop, pulling that prop control back to the low-speed position can dramatically improve glide performance — like taking off the speed brakes! Keep the ball centered to avoid any altitude-robbing slips or skids, and adjust elevator trim to maintain best glide speed. Without proper trimming, our airspeed is likely to wander, and our glide will be shortened.
Wind also has a dramatic effect on our glide distance, so a viable landing site to windward will need to be closer than one that is downwind — at least to an extent.
Turns will have the biggest negative impact on our glide — the steeper the bank, the greater the effect. In most cases, a bank of 30 degrees to 45 degrees should be just about optimum for making turns needed to line up for a landing. Lower bank angles mean we spend too much time turning, meaning we lose more altitude. For bank angles greater than 45 degrees we pay a steep penalty in altitude loss and risk of overbanking and loss of control.
One way to verify this for ourselves and build the needed skill to deal with such an emergency is to practice (at a safe altitude!) power-off 180-degree turns. We can try varying the bank angle of the turn and see which yields the least altitude loss in the maneuver, and then practice power-off landing in the pattern to get the sight picture firmly engrained.
Going the distance isn’t just an expression that we use; it’s a potentially life and death proposition for a pilot faced with an unexpected situation or an emergency. The better we are at extracting performance from our aircraft, the more likely we can succeed when a situation goes awry.
Robert N. Rossier, EAA 472091, has been flying for more than 30 years and has worked as a flight instructor, commercial pilot, chief pilot, and FAA flight check airman. For more from Robert, see his Stick and Rudder column each month in EAA Sport Aviation.