Friday, October 1, 2010
The Ever-Stylish Hover Check
Within U.S. Coast Guard operations, as another exception to the rule, we trained to accomplish no-hover instrument takeoffs, with feet flat on the deck using yaw hold AFCS functions, so that flying cleanly away from runway surfaces, yet dead on centerlines, could be accomplished safely during categorically zero/zero weather. (We practiced this maneuver often, but only rarely used it in actual operations.) But most of the time, across most of the imaginable spectrum of helicopter departures, pausing in a fully stabilized hover with a three-foot skid height for several seconds of introspection before committing to the hazards of full amplitude flight is a profoundly worthwhile enterprise.Bringing a helicopter to a cautiously deliberate hover in ground effect and observing it there for a few moments can accomplish:
• Confirmation of the generally correct functioning of drive train and rotor systems at flight speed and high torque values (not a bad choice of a set of parts and functions to trust but verify);
• Proper response of the flight control systems to control inputs (nothing beats actual proof testing for confirming mechanical performance);
• Correct and accurate reporting of engine and flight instruments, specifically to include heading cards and attitude gyros (seeing here is truly believing);
• Checking power required to hover in ground effect (this is a good number to know, since the weight of the helicopter,
and this power required, will be decreasing for the remainder of any given flight leg, and the hover check values represent something like a worst case scenario for the present segment);
• Noting the power available for either a conventional or a vertical climb to continue the takeoff (hovering, of course, typically requires more power than cruising, but the differences between hover check torques, manifold pressures [for piston power], compressor or engine speeds and temperatures, and maximum allowable values for these parameters, will provide a useful perspective on expected maximum takeoff performance);
• A great, practical confirmation of data already known about aircraft weight and balance (together with the above mentioned confirmation of power margins, the way the aircraft “hangs” under the rotor mast can be immediately observed, and is, after all, the final real world evidence for satisfactory geometric airframe weight balance);
• Confirmation that no obstructions or impediments to flight threaten to interfere with the free departure of the airframe (has anyone ever heard horror stories about tie downs, wash hoses, fueling nozzles or electrical lines being forgotten or overlooked, literally connecting the helicopter to impending disaster?); and
• Witnessing the engine(s) and drive train sustaining a high power output through six to eight seconds of hover check flight can be thought of as statistically meaningful in terms of systems reliability (if power systems can be demonstrated to deliver high ranges of output through a generously prudent hover check, it can be considered statistically likely that fuel control and power delivery systems are working correctly, and this is very nice to know).
I know of one particular quick departure that was intended to be executed as an immediate climb from a parking ramp, over power lines and a densely obstructed railroad track area, but by random chance was held for a few moments by a tower controller who turned out to be, in the annals of hover check lore, an angel. As the pilot waited a seemingly interminable few seconds, stuck in a low hover and impatient for release, his single turbine spun unceremoniously to ground idle.
The helicopter, of course, auto-rotated to the deck, safely preserved. A maintenance check revealed that a Py air line connector, necessary for proper fuel control operation, had become loose, instructing the engine to power down. If the departure had proceeded as planned, the outcome of this episode would likely have been dramatically unfortunate. But a good hover check—however unintended—literally saved the day.
Let’s use stories like this one, along with the consideration list started above, to remind ourselves that a few seconds spent confirming that our aircraft machinery is happy and well is a wise investment indeed. Make sure that hover checks always remain fashionable in your operation.
After all, the art of safety should never be allowed to go out of style.