Professional Pilot, January 2019

cally extend the slats at high AoA thus preventing a stall Now I need to stop here and say that I do not in any way suggest the pilot lower the flaps or slats at the onset of any stall Full throttle s or power levers for our French colleagues and decreasing AoA should be the pilots immediate actions Clean stall happens at higher airspeed The latest version of the ATP Practical Test Standards PTS clearly emphasizes that it does not matter which action is taken first in a stall recovery such as more power or reduction of AoA However hopefully pilots recall that in simulator training a clean stall happens at a higher airspeed than a stall with the landing gear down and full flaps I do recall flying an F 15 in a slow speed dogfight extending the flaps immediately forcing the opponent out in front assuming I beat him to the punch on lowering the flaps and achieving an offensive position Why Because lift just increased by changing the camber of the wing allowing the pilot to trade airspeed for altitude and causing the faster flying jet the opponent to shoot out in front of my jet Value of a stick shaker In some corporate or business jets the yoke has a stick shaker which vibrates and shakes at high AoA typically about 080 on a scale of 0 to 1 In the military we called this the tickle 52 PROFESSIONAL PILOT January 2019 where your wing was performing at L Dmax or maximum differential between lift and drag for your wing The pilot can actually feel a slight vibration on the stick or yoke while in a turn or increased AoA when flying at L Dmax All new pilots should have felt this in slow flight during their private pilot training In the F A 18 Hornet I remember flying at 30 degrees AoA on the edge of a stall at about 90 knots trying to out gun the brains of the poor fellow who was my opponent in his gunsight But I was still flying even though technically in a stall at this higher AoA Low speed buffet at high altitude Some pilots might have experienced the low speed buffet at high altitude when making a standard rate turn at FL400 at maximum gross weight The onset but not prolonged duration of the low speed buffet can be equated with that tickle feel and maximum lift over drag differential The relationship between L Dmax and CLmax can be appreciated in Figure 5 When does the stall begin and end When does the stall begin and when does it end The recent changes to stall prevention and recovery in the PTS do not quantify when the stall ends Is it below the 080 AoA index in Figure 5 which the stick shaker comes on Is it when the stick shaker stops shaking What if there is no stick shaker or AoA gauge in the jet How does one define a successful stall recovery The response so that a recovery does not incur a secondary stall is not sufficient in my opinion A quantitative not qualitative response is needed here Figure 6 shows how CLmax varies with changing Mach numbers This might be intuitive as pilots may have experienced the low speed buffet at a higher Mach number at high altitude but not at sea level at that same Mach number Figure 6 also shows that with lower Mach number the ability to fly at higher AoA increases Notice in Figure 6 that CLmax at Mach 080 occurs at a much lower AoA than at Mach 040 This helps explain why pilots experience the low speed buffet at high altitude versus at lower altitude at the same Mach number Turning at a standard rate like 3 degrees per second at high altitude will cause the pilot to encounter the slow speed buffet earlier as less lift is now available when in the turn In summary especially with new wing designs on aircraft being developed using updated airfoils flying at higher AoA but below CLmax may mean the aircraft is technically not in a stall but technically still capable of flight Knowledge of what AoA is being flown at all regions of the flight envelope is the key to safe flight Relationship between L Dmax and CLmax L Dmax occurs at a lower AoA Change in CLmax with Mach number Michael Petridis is managing director of Standard for Excellence in Business Aviation SEBA Council and also a partner at VIP Jets an aviation consulting firm in Dallas TX Fig 5 STALL Angle of attack degrees CLmax CL CD CD L D L D L Dmax CL Fig 6 OL M 8 7 6 5 4 0 CL α α