PSI - Issue 36

M. Karuskevich et al. / Procedia Structural Integrity 36 (2022) 92–99 M. Karuskevich, T. Maslak, Ie. Gavrylov et al. / Structural Integrity Procedia 00 (2021) 000 – 000

93

2

monitoring for small planes as well. Taking into account the limitation of the weight for this category of aircraft, the described here miniature autonomous fatigue indicator looks a prospective alternative to the complex monitoring systems. Another problem of light planes is an operational overstress of principal structural elements. Many of light planes have very much limited set of instruments onboard. As a rule, the light aircraft are not equipped with a flight data recorder or a cockpit voice recorder. Sometimes these planes are not equipped with an accelerometer. At the same time, the planes of this category suffer in the biggest extend from the action of inexperienced pilots, those exceeding limitations of speed, rate of turn, etc. These planes are not equipped with systems to compensate action of wind gusts and other factors causing an unexpected increase of load factor. The load factor range for these planes is defined by correspondent flight envelope. For example, for Cessna 172 flight load factor (flaps up) maximum is +3.8g, the minimum load factor is -1.52g. Overstress of the plane components due to the incorrect flight regime can lead to the primary structure deformation, and even to the aircraft crash. The absence of accelerometer onboard leads to the question concerning the role of excessive acceleration in the process of damage or crash. One fatal accident may be considered as an example (Aviation Investigation Report, 2001). The Cessna 210L Centurion in flight break-up happened in June 2001. The case was investigated but the final precise decision regarding the cause of the aircraft fracture was not made. The detailed examination of the wings revealed no signs of flutter or pre existing structural weakness, such as fatigue, corrosion, or loss of rigidity. The direction of the deformation in both wings indicates that the aircraft was subjected to a severe positive g load factor. Possible causes of an overstress condition include aircraft manoeuvers, gust loads, or a combination of both. The presence of the overstress indicator would clarify the situation and develop some recommendations for pilots and engineers. The key points of the methods were considered early in the work (Karuskevich et al., 2020). Detailed explanation presented below is focused on the practical application in General Aviation. 2. Light aircraft fatigue Like any aircraft, the LSA are subjected to a wide spectrum of operational loads. The level of allowed acceleration for the LSA is higher than for the planes of transport category: the maximum positive load factor prescribed, for example for the Viper SD-4 (TomarkAero) is equal to +4.0g, the minimum is – 2.0g. The forces and moments, repeated many times during the flight or even one time during the flight, but many times during the aircraft life span, cause accumulation of the fatigue damage in metal components. Among the loads on the primary structure, the attention deserves the aerodynamic forces, gusts loads, motor vibration, landing loads, loads caused by the unevenness of the airfield. Analysis of the fatigue problem for small planes has shown that the metal fatigue problem exists and methods for early diagnostic of fatigue damage to prevent structural failure are demanded. As a proof of the necessity of the special attention to the metal fatigue for small aircraft, the fatigue damage case study of the Piper aeroplane primary structure may be considered. During a routine inspection of a Piper Model PA-46-350P aeroplane, 1.53-inch (3.88 cm) fatigue crack was discovered in the main spar lower cap, Fig. 1. The crack discovery was at 5,273-hour time-in-service (TIS) versus the type certificated life limit of 15,580 hours TIS (Aero-news, 2015).

Fig. 1. Fatigue crack in the main spar lower cap: damage area inside the wheel well; detail view of crack (Aero-news, 2015).

The importance of the fatigue problem for another plane, this time Cessna, confirmed by the Federal Aviation Administration (FAA) proposal to adopt an airworthiness directive (AD) affecting numerous models of Cessna 172,