by Wg Cdr P K Raveendran, SC (Retd.)


Aerodynamic Flight Path Gradient (γa) is an important and critical performance parameter, often limiting the operational exploitation of the aircraft. For example, the maximum takeoff weight of a multi engine civil transport aircraft on a given day (pressure altitude and ambient temperature) is limited by the legal requirement of a minimum second segment climb gradient (γa) of 2.4% with critical engine failure at the decision speed (V1). Therefore accurate determination of this critical flight performance parameter especially during certification flights is crucial. To overcome the limitations of the existing indirect methods to arrive at γa in flight tests, the author developed a simple, direct and elegant algorithm to determine γa using the on-board air data parameters. Based on this, a special flight test instrument was designed, calibrated and flight tested to prove its efficacy. This was done as part of the Flight Test engineers’ (FTE) thesis work in partial fulfillment of the requirements for award of FTE Brevet at the French Flight Test School, Ecole du Personnel Navigant d’Essais et de Reception (EPNER) during 1979-80. The algorithm has since been used in the performance flight testing of transport category aircraft, e.g flight testing of re-engined AN-32 aircraft at ASTE, Air Force.

In the case of the Light Combat Aircraft (LCA) Tejas, as a precursor to provide care-free maneuvering capabilities for the Final Operational Clearance (FOC), there was a requirement to provide reliable low speed warning computed as a function of CAS, Aerodynamic Gamma and configuration, for interim Initial Operational Clearance (IOC). For this purpose the Control Law (CLAW) team was computing Aerodynamic Gamma indirectly from the on-board simplex Inertial Navigation System (INS) and flow angles (α, β). INS being a simplex source prone to errors/failure at steep attitudes the CLAW team was looking at options like integrating a duplicate INS on-board for mechanization of this safety critical low speed warning. At that point the author suggested use of his algorithm to compute Aerodynamic Gamma on-board with adequate accuracy and redundancy which has since been done and the aircraft attained IOC on 20 Dec 2013. The impact of using the algorithm on the cost and time saved for this prestigious national program is very significant.

Key words: Aerodynamic Gamma, Inertial Gamma, Second segment climb gradient, Low speed Recovery (LSR), Light Combat Aircraft, Tejas, Initial Operational Clearance (IOC)

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