Humans like to push limits. Many have found themselves coasting into the next gas station on fumes, or worse, on the side of the highway. Sadly, this is the same mindset we can fall in to with downwind takeoffs. “I had no problem with a 5 or 6 knot tailwind takeoff last time,” or “I’ve taken off with a 10 knot tailwind. I don’t know why another 5 knots would hurt anything.” You get the point. “Permissible” downwind takeoff limits have often been debated. After all, the only thing two helicopter pilots can agree on is what the third one is doing wrong.
Our self-rationalization can get us in trouble in a hurry. What was a 5 knot tailwind takeoff one day will build progressively until you “accidentally” find out just what that tailwind limit is! I’m not implying that a 3 to 5 knot tailwind takeoff will get you hurt or killed. What I am saying is don’t fall prey to that “I’ll just go a little more this time” mentality that has been known to find its way inside helicopter cabins. It exists and sadly I see it more frequently than I care to admit.
If a picture is worth a thousand words a diagram is worth a thousand explanations (or at least one). Let’s take a look at the mechanics of downwind takeoffs from a technical, yet practical explanation with a basic graphic representation.
Looking at this generic diagram we see three different helicopters each with a certain amount of power being used depending on the airspeed of the helicopter or the relative wind the blades are utilizing. At first sight of the diagram it should remind you of a basic power curve diagram and the fact that our wonderful machines are the only vehicle known to man that take more power to go slower. The power required curve could represent TQ (torque) required for a turbine helicopter or MP (manifold pressure) required. You will see at the bottom of the power required curve we have the “bucket-speed” or the speed at which we get the greatest airspeed for the smallest amount of power required. This “bucket-speed” area should be familiar as it is normally the best autorotative speed range as well. Looking at Helicopter #1 we see a helicopter at or near max power while in a 0-airspeed hover; in or out of ground effect, it makes no difference for this explanation. Granted, it will not always take max power to hover but consider Helicopter #1 at or very near max power for this explanation. Following along with the example helicopters you will see that helicopter #2 now has 15 knots of forward or headwind airspeed and the amount of power required is substantially less than the power required for that 0-airspeed hover. This concept in and of itself is no surprise (or shouldn’t be) to even the most novice students. It is helicopter #3 where we can get into trouble!
Looking at helicopter #3 we see that we have 15 knots of reward or tailwind airspeed. Looking at the power required we see that it is a mirror image of the power required for helicopter #2. It takes the same amount of power, in theory, to hover with a 15 knot tailwind as it does a 15 knot headwind. If you do this bring your tap shoes because you will be dancing on the pedals. (For the sake of aerodynamic argument tail rotor authority and increases in power required with use of the tail rotor are excluded from the equation.) Another way to look at this explanation is that the blades don’t care where the 15 knots of wind is coming from; in essence, with a 15 knot tailwind you could visualize the retreating and advancing blades (as you know them to be) have essentially traded places. I’m certainly not telling you to make a habit of hovering with a tailwind! A host of factors dictate why you shouldn’t, including loss of tail rotor effectiveness issues; yaw stability; longitudinal stability issues due to wind getting under (or over) large stabilizer surfaces; and potential TOT and compressor stall issues in turbine machines.
So, if we have a 15 knot tailwind as seen with helicopter #3 and we commence a downwind takeoff the rotor system is starting with a minus 15 knots of “support,” and therefore must outrun the tailwind and lose the translational lift that it had while stationary. Guess what? That takes more power! Essentially by taking off with this 15 knot tailwind you must use the power necessary to reach the power required area of a 0-airspeed helicopter as we described with helicopter #1. At this point you have a ground speed of 15 knots but the rotor system is experiencing a forward relative airflow of zero; you are getting no help from translational lift, and soon the helicopter will begin to descend. Remember where you are at this point; at or near max power. With the helicopter sinking you add more power, which increases the need for tail rotor robbing you of even more power. This is why I referenced “at or near max power” above. If you were faced with this situation, heavy, and in less than ideal performance conditions you may not have enough power and pedal to get you “over the hump” of the zero airspeed point. This dangerous and often overlooked downwind takeoff condition sets the table for a hazardous cycle.
While many have fallen prey to pushing the limit with the low fuel light in their car, one must realize that pushing the limit with downwind takeoffs can lead to disastrous results. We must resist the temptation to gradually increase our accepted risk level regarding downwind takeoffs. Obviously with the right power margin and ideal conditions taking off with a certain amount of tailwind speed gradient is possible and can be made safely. It is human nature that we must avoid.
As always, I may be alone, but I doubt it. What say you?