Page 12 - Nov 2015 Volume 9, Number 11

AOARedux(Aspen’s Angle OfAttack Solution)By Adam AlpertFrom the NTSB Files:L A X 8 7 L A174File No. 245 04/12/1987 VENTURA ,CA Aircraft Reg No. N32BB Time (Local): 14:03 PSTACCIDENT SEQUENCE:Sequence event #1: LOSS OF CONTROL – IN FLIGHT Phase of Operation: DESCENT – NORMALFindings1. (F) THROTTLE/POWER CONTROL – EXCESSIVE – PILOT IN COMMAND2. (C) STALL – UNCONTROLLED – PILOT IN COMMAND3. (F) REMEDIAL ACTION – DELAYED – PILOTIN COMMANDSequence event #2: DITCHINGPhase of Operation: DESCENT – UNCONTROLLEDFindings4. (F) TERRAIN CONDITION – WATER,ROUGHSequence event #3: FIREPhase of Operation: LANDING – FLARE/TOUCHDOWNFindings Legend: (C) = Cause, (F) = FactorThe Grumman Widgeon stalled at 200 feet above the water, with only a small buffet from the wing to signal the event. The pilot, cognizant that the amphibian’s design excluded any kind of stall warning device, constantly monitored his airspeed – especially when low – to mitigate the risk of a stall. And yet, even with the airspeed indicating above-stall, a classic stall/ spin event was unfolding. The pilot was quick to apply power and lower the nose, but his actions came too late, the altitude was just too low. The Widgeon impacted the water, left wing first, with such force that a post-crash fire ensued. The pilot was seriously injured, along with one of the two passengers. The airplane eventually sank and was never recovered.This unfortunate accelerated-stall accident occurred during a photo shoot of a sailboat race. The physics in play that day apply to many loss-of-control accidents, one of the leading causes of GA fatalities according to the General Aviation Joint Steering Committee. How can this be? From day one of their training, students are drilled on the disastrous outcomes of stall/spin accidents, and how to prevent them. Furthermore, stall recognition-and-recovery is a big part of recurrent training, especially in Part 142 schools. And still, these accidents continue to occur.The explanation is largely rooted in an incomplete understanding of the dynamic nature of lift required to maintain controlled flight. While airspeed is a significant factor, it alone is a relatively poor predictor of lift. And yet, airspeed is the primary metric that most GA pilots lean on to assess their margin and predict how close the airplane is to stalling. To more precisely determine how much of the wing’s lifting capacity is consumed, the wing area, weight, temperature and altitude (for establishing air density), load factor, and center of gravity must all be included in the calculation.10 • TWIN & TURBINEA much better indicator of lift capacity is the Angle of Attack (AOA), the angle between the wing chord and the relative wind. This angle precisely correlates with the coefficient of lift (CL), the inverse of which is the lift capacity remaining. Knowing the lift capacity remaining means knowing how close the airplane is to a stall at any time (CL Max). As CL increases toward CL Max, the lift capacity remaining decreases, reducing the margin of safety. At CL Max all the available lift capacity is consumed and the airplane is at risk for a stall, potentially leading to a loss of control event. Conventional AOA instruments, typical of airliners, business jets, and military airplanes, monitor such conditions as air pressure, temperature, weight, CG, and load factor, and often employ a complicated exterior vane sensor that literally measures the angle. These systems work well, but they are expensive and difficult to retrofit, largely the two reasons they have not enjoyed much success in the single-engine piston and twin markets.A New Day For AOAFortunately, change is in the wind. A new series of GA-targeted AOA products has recently emerged,JANUARY 2016