PC-12 Protection System

DESIGN PHILOSOPHY - STALL - REQUIREMENTS To cope with the extremely lar ge center of gravity A UNIQUE range from 13%mac most forward to 46%mac most The FAR 23 certification requirements for stalls rearward, the PC-12 was designed to utilize the The market dictated the design dictated that the maximum stall speed in the power advantages of a T-Tail. philosophy of the PC-12 in that off landing configuration may not be greater than it should be designed and 61 kcas. It further dictated that the stall must be STALL CHARACTERISTICS certificated to F AR Part 23 characterized by either an "uncontrollable (single engined). It should offer downward pitching motion of the airplane" or by From the start the PC-12 could meet all stall similar or superior climb, cruise the control reaching the stop. requirements, indeed the stall characteristics of the and payload performance to PC-12 at low thrust coefficients were benign. turbo-prop twins, similar or For wing level stalls, it was then required that However, with a power setting of 75% MCP the superior comfort, operational "during the recovery part of the maneuver , it must PC-12 stalls at a speed of 45 kcas, a pitch attitude flexibility in terms of short field be possible to prevent more than 15º of roll or yaw in excess of 30º nose up. In this extreme condition operation, cargo door and floor by normal use of the controls". For turning flight as standard, single pilot and accelerated stalls, "it must be possible to operation, capable for operation regain level flight without exceeding 60º of roll in under Instrument Flight Rules either direction from the established 30º bank". (IFR), at night, and flight into Compliance had to be shown in all known icing conditions. It configurations and with engine power up to 75% should offer superior safety in of the maximum continuous rating. terms of systems reliability , crash worthiness and should Furthermore, § 23.207 (a) stated that "there must possess safe and pleasant be a clear and distinctive stall warning, with the handling characteristics. It flaps and in any normal position, in should also offer significantly straight and turning flight". reduced acquisition and operating costs. In order to achieve the project specification for a the stall is finally characterized by a wing drop 265 ktas cruise speed and nevertheless fulfill the which at such low speed the do not have W ith such a stringent design requirement of a 61 kcas Stall speed, a small wing the authority to counteract. requirement, the PC-12 was with a high lift profile and Fowler flap system was always going to be a unique designed with an extremely high lift coefficient. Furthermore, in all conditions the elevator in the T - aircraft. It is the largest single This resulted in the selection of a wing profile with T ail arrangement maintained suf ficient aerodynamic engined aircraft currently being a very "peaky" lift curve at its maximum point. authority to control the aircraft in pitch throughout built. It has an operating the stall. The requirement of full back stick or speed envelope from a "uncontrollable" nose down pitch motion could stalling speed of 61 PC-12 Lift Curve Characteristics not therefore be fulfilled. In conclusion, the PC-12 Gear Down, Flap 40 kcas to a cruising speed 3.5 'power on' stall characteristics exhibited: of 265 ktas at FL 250, an 3 extremely large center of • No pre-stall warning gravity range from 2.5 Power On • No pitch down 13%mac most forward 2 • W ing drop > 15º to 46%mac most Power Off 1.5

rearward, and an engine Lift Coefficient Characteristics which were aggravated with capable of producing 1 increased flap, power and rearward CG. 1,200 shaft horse power . 0.5 The PC-12's high 0 STALL SOLUTION power, low speed yields -15 -10 -5 0 5 10 15 20 25 a very high thrust (Deg) Flight tests were supported in parallel by a further coefficient at the stall. series of wind tunnel tests in an attempt to find This was a feature which aerodynamic solutions, many of which were created unique challenges successfully tested in flight but were considered during development flight unacceptable at the relatively late stage in the testing, in particular stall testing. (continued on page 21)

20 (continued from page 20) program. Finally it was decided to prevent the stall by a strong shaking of the control wheel; if he W e hope this enables you to aircraft from reaching the aerodynamic stall by nevertheless continues to pull and reduce speed, better understand the roll of the introducing a stall protection system (/ the stall protection system is activated and pushes stick shaker/pusher system on pusher). The FA A Advisory Circular 23 8A gave the control wheel forward. The aircraft pitches the PC-12, and we wish you indications about the operation of down and immediately resumes normal flying, with continued safe flying. systems, this information was amended by the moderate altitude loss (typically 150 to 200 ft). Swiss FOCA. The final requirements becoming: Leonardo Manfriana A significant amount of development and Chief Aerodynamicist • 15º< W ing Drop < 40º Natural certification testing was performed throughout the aerodynamic stall to be protected by 2 kts extremes of the weight & balance and altitude Tony Taylor • W ing Drop > 40º Natural aerodynamic envelopes during which over 1,000 stalls were Head of Flight Test stall to be protected by 5 kts conducted. In addition a significant amount of abusive type manoeuvers (Bunts to 0g, Roller • Operation of the stall protection system coasters and flight in turbulence) were also Pilatus Aircraf t, Ltd. to be mandatory for flight investigated in an attempt to inadvertently activate P.O. Box #992 • In all cases, stall warning (shaker & aural) the pusher . The system was also tested during 6371 Stans, Switzerland shall operate "at a speed exceeding the stalling natural icing conditions. +41 41 619 64 25 speed by not less than 5 knots and by not more www.pilatus-aircraf t.com than 10 knots or 15% of the stalling speed". It is possible to override the stall protection system by holding the control wheel back with sufficient In order for the stick pusher to operate with force (approximately 50 pounds), or even to sufficient margin from the natural aerodynamic stall disconnect the system. In this case, the aircraft is and still meet the Stall speed rrequirement of 61 obviously no longer protected against an kts, the wing span and flap span were increased aerodynamic stall; depending on the power setting by a total of 2 meters. This modification required a and flap configuration, severe (although not completely new wing to be built, which could only unrecoverable) roll departures can occur .This is be tested on the second prototype (P02) for the of course very dangerous in the initial and final first time. In the meantime all stick pusher flight phases, since even a very experienced pilot conceptual testing was developed using the first will not be able to recover from a critical prototype (P01). The stick pusher grew in aerodynamic stall departure without a severe complexity throughout the development process altitude loss (up to 700 ft). and is now a sophisticated system using modern sensor electronics and qualified according to the CONCLUSION latest Failure Mode and Criticality Analysis (FMCA) methods and requirements. The PC-12 is unique in that it is the only aircraft of its class to be protected by a stall protection The system consists of two digital computers, two system, as normally found only on much heavier vane-type A O A sensors installed on the left and and more sophisticated aircraft. The system, right wing, a single stick shaker , a single stick developed by Pilatus, was certified through an pusher actuator and caution devices. In extensive flight test program and has been proven correspondence to specified A O A values, each in operation on more than 500 aircraft and more computer gives a shaker activation, pusher than 850,000 flight hours. It provides enhanced activation or pusher deactivation command to the flight safety and peace of mind by preventing the actuators. The system is set up in a way that either pilot from stalling the aircraft under any flight computer can independently activate the stick condition, without any significant negative effect shaker and audio warning, but a signal from both on aircraft performance. computers is needed to activate the stick pusher . To maintain adequate speed mar gins between stick It must be emphasized that if the stall protection shaker , stick pusher and natural stall in all system is deactivated, there is a very real risk of conditions, the A O A values that trigger the stick incurring an aerodynamic stall with potentially shaker and stick pusher actuation depend on flap severe roll departures, particularly in the flaps position and, with 40º flaps, on engine torque. down configuration with high power. When the airspeed is reduced below the stall warning limit, the pilot is warned of the impending

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