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A rotorcraft tale: toll of a tailwind.


Loss of control (LOC) is the common element of several civil and USN/USMC rotorcraft accidents over the last few years--each involving situations where the power required to fly exceeded the engines' power available, or "oomph," to do it. There were well-trained, experienced and qualified crews at the controls in each instance--all aware of the effects of higher temps and thinner air on power margins. Entry into unsustainable flight regimes were encountered with each event ending in significant airframe damage and some loss of life.

The laws of aerodynamics are what they are, and keeping an eye on the OAT/FAT, the pressure altitude, the torque gauge, and the trusty NATOPS pocket checklist has traditionally been the key to success in coping with each scenario. Today, with multi-sensored instrumentation and performance-paged multi-function displays (MFDs), keeping the bird airborne and at full flying potential is surer and safer than ever. But, stuff does happen.

One time-honored ingredient is factored in your favor on every landing aboard ship, and in virtually every touchdown to a controlled strip on terra firma. It's a factor you know you need working for you, and one you'll consider in every maneuver you make. Yet, aside from some super-sophisticated systems, it is often an essential element only available via "eyeball" cue, and at times it'll put you behind the power curve without a clue. It may not have been the culprit in each of the cases we'll discuss, but there are clear grounds to suspect that it could have been. Got it figured yet?

Consider a civil registered, single-rotored, electronic news-gathering (ENG) chopper, down close to the surface in an urban area. They have a few places for a safe touchdown in the event of "stuff." The bird isn't heavy and the air isn't hot, but its mission has it hovering on a blustery day, in the sights (camera) of another ENG bird nearby. The camera records a low, slow and troubled transition from a hover to forward flight, apparently in an attempt to make an emergency landing. That transition winds up in another hover, in which yaw control is lost. The pilot has apparently encountered some form of loss of tail rotor effectiveness/authority (LTE/LTA). During the attempt to recover, the bird continues it wobbling ways, eventually striking a building and shredding itself. Somehow, all aboard are spared. Post-crash analysis indicates no problems with the engine, rotors or control linkages.

In another mishap involving a close kin to a USN workhorse of several decades, a civil aircraft had made several landings at high altitudes, under hot, remote conditions through the afternoon and into the early evening. As is often the case at such sites, near-vicinity metro data was hard to come by. However, the pilots had completed preflight-performance calculations, which indicated liftoff from the frequented, high helo site was within the aircraft's hover-out-of-ground-effect (HOGE) capability. With the helo pad on uneven terrain, it was clear they'd fly out of ground effect during departure--before they got much help from effective translational lift (ETL). However, with early evening temps and cooling, their earlier experience with the site, and their calculated safety margin, they expected a safe departure. Though all systems were go, it didn't work out that way. While inaccurate reporting of aircraft weight was a complicating factor, analysis by the aircraft manufacturer pinpointed another, and possibly pivotal factor.

There's at least one element potentially common to all these mishaps. During transition to forward flight, today's prop-rotor center of gravity (CG) shifts and cross-coupling challenges are not encountered by traditional rotorcraft. When the aircraft is morphing from the rotor-to-wing mode, the wing becomes the sole source of lift as the nacelles transition to the airplane configuration.

With the wind on the nose at 15 knots and gusting to 27, and groundspeed barely over (no wind) translational lift, the flow over the wing--the sum of wind and ground speed--could be near 50 knots. Not only is the wing producing several hundred to a few thousand pounds of lift at that point, but the proprotors' induced power (the power required to overcome induced drag) has the total power required on a slide down the "back side" of the power-required curve (Fig 1). At this point, climb capability is rising rapidly, and the challenges of CG shift and thrust-pitch cross-coupling are diminishing. However, a quick 180-degree heading change, tail to the wind, will spoil wing lift and put the rotors back below ETL. With nacelle transition underway, the nose will pitch down, as the required power rises rapidly. An uncontrolled descent quickly develops, and at IGE (in ground effect) levels over flat terrain, an impact is virtually assured. If not earlier, by now you know that pivotal factor--the potential common element of each of these mishaps was wind direction.



In the first instance (ENG helo), winds in the vicinity were reported at 14 gusting to 20 knots. The "footage" from the observing ENG helo camera appears to show strong gusts in the direction of flight. A helo under otherwise demanding conditions will "back up" over the high point of the power required curve (Fig 2) while attempting to slow to a "stop" over the ground, in a tailwind. If the available power was marginal to begin with, a rotor "droop," and potential LTE/LTA are likely, as the attempt is made to decelerate to zero ground speed, with tail-to-the-wind. Similarly, a successful (marginal) hover in a tailwind might very well result in the same droop-LTE/LTA as the aircraft flies "up" the power required curve (Fig 2), when accelerating "with the wind" from the downwind hover into forward (ground speed) flight.

You'll remember that the USN workhorse mishap occurred in a dry, high, remote area, with no metro nearby. While the pilots knew the OAT and the pressure altitude at the pad, their (higher-hotter) calculations were based on a no-wind condition. They had no cues by which to make a good guess at the winds, but the ground crew had tried to assist with ribbons tied in the lower branches of nearby trees, and guessing the winds on the tail were zero to six knots. The hover power-check went as previous, and the departure over treed, down-sloping terrain was to be a repetition of the earlier, warmer ones that day. The post-mishap analysis by the manufacturer determined that a five-knot tailwind would have caused a 3.4 percent overestimate of the (constant-power) thrust capability in a no-wind condition. The difference in ground-effect benefit from the point of the hover check (solidly IGE) to the point of the first tree strike (marginally IGE) was another thrust decrement (constant power) of 13.4 percent. The power required to produce hovering thrust was at least 200 horsepower higher just before the first tree impact than it was where the power check was conducted seconds before. In this example, and using Fig. 3, power required is shown by the red curve, and power available by the yellow line. Power-wise, a 13-knot tailwind feels like 12 knots on the nose (see dashed black line). But with the headwind, power required decreases with forward stick/ground speed, while it increases with forward movement, if the wind is actually on the tail. The aircraft "sees" excess power (climb) with forward stick/groundspeed, but with the tailwind there is a power deficit (descent) with forward stick/forward movement. What are the implications during a marginal-power approach to a hover with a tailwind?


There is more to be known about each of these accounts, but in each case the point should be clear: If you haven't found yourself launching or landing in the unknown outback, with a limit-pushing load, you likely will someday. If so, take a good look at the lay of the land, and consider that the air may be moving. If on your nose, it's a "breeze," but on your tail, there will be a toll.

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Author:Cress, Jack
Date:Jan 1, 2014
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