MAFT -The Tejas Airframe tests                                                                             Prof. Prodyut Das

It has recently been announced that the main airframe of the Tejas is to be fatigue tested. The programme is named MAFT – Main Airframe Fatigue Test. This is welcome news not only because it is a very necessary test but it is probably an indication that the Tejas airframe is finally in a reasonable state of fitness to be tested. A figure being bandied around is that the airframe will be tested for a life of 10,000 hours. This may be an international norm but unless justified by the statistical data on the actual life experienced by single engine fighter in various roles in teh IAF it is both unnecessary and wasteful, particularly of time. We must learn to gently break rules.

The comment about the airframe being finally ready stems from the information given by officials and that showed that the aircraft was probably grossly over designed. As revealed by the “undercarriage incident” mentioned in Blue Skies podcast SE – an undercarriage came down at 700 kmph under condition that made the “q” loads that were three times above the limit load. The undercarriage should have ripped off. Whilst that particular pilot’s wife is grateful for such overdesign it could not have done much good to the aircraft performance. The aircraft had acknowledged serviceability problems due to wrong location of various items requiring service attention. As I have discussed elsewhere the Tejas seems to have suffered from poor weight control the gross of which was an empty weight of 7200 kilos for an aircraft significantly smaller than the F 404 Gripens .   Obviously subjecting such an airframe apparently in need of refining to lengthy test did not make sense but did the design team rest on its laurels? Did we perhaps miss an opportunity?

Subjecting the airframe to load tests is of course an integral part of aircraft development. Usually against an initial order for four to six airframes at least one is set aside for structural tests. Initially the load is the ultimate design load with suitable safety factors. For metal aircraft the factor used to be 1.5 times and for composites because it was more difficult to control quality it was 2. So much of the theoretical advantage sof the composites get nullified in practice.

It is not sufficient if the load test is passed without failure because that gives no indication if the structure is over strong. The test must also be designed to show up that the aircraft can only withstand the design load but not a kilo over that. Chief Designers sweat over that aim. Legend has it that the HF 24s wing, designed in the days of wooden slide rules, failed at 1.01 times its failure load! Much is known about so many milestones of the Tejas project but I cannot recall information of the load test. It must have passed the load test and the aircraft was certified- that is procedure. There is no information if subsequently it was extensively worked to achieve the lightest structure. Notes please will be welcome addition to knowledge.

One caution in load testing is that poor detail design can cause a structure to be overweight and yet fail in the load test. I witnessed that happen in an all-composite GA aircraft. Though overweight, it failed at a point where the fuselage pod tapered sharply to a boom. Aerodynamically the “sharp” junction attracted the eye. There was a sense of shock- which may have been excusable- the structure was overweight- and “loss of face” which was less understandable because the test should have been designed to explore the failure margin for refining the design. 

Regarding the value of airframe life of 10,000 hours obviously there must be documents that state such a figure but we have to think somewhat differently and break rules gently where they do not make obvious sense. We have to question how much does this figure apply to us. As young gentlemen during our NCC visits to KKD the anecdotal figures I have for the Hunter was that it would go for an overhaul after 1000 i. e. about 5 to 6 years by current expected rates of usage. Perhaps the interval was longer then. When asked what happened after the next 1000 hours the “Chiefie” said very few Hunters survived 2000 hours. Many of the MiG 21s when flown in the close support role the figure of airframe life was 700 hours before the thing crashed though I cannot now remember  where I got that figure. For the MiG 27, and I am on much surer ground about this, there was much “to do” about extending the airframe life from 2200 hour to 2500 hours where I attended a few meetings. At that time the MiG 27 had been in service for 20 yrs! Despite superior modern materials flying conditions in India are so different. My view is that a single engine close support fighter will not last 10,000 hrs or can be kept serviceable so our own data has to be analysed and our own standards laid down.   

The bishop’s one-horse shay

In the well- known poem by Oliver Wendell Holmes “The Bishop's one horse shay” was designed for every part to last one hundred years. The poem tells us what happened. Aircraft are not designed like that and much of the airframe can really last for a “long time” i.e. when the aircraft is phased out or becomes unserviceable due to non structural reasons . In aircraft focus has to be on about seven areas – the cockpit bulkheads and the cabin canopy is one of such seven -and the rest of the structure usually looks after itself through good design practise. In an earlier age the testing was rather “lumpy”-the usual stressed areas were stressed on a” Christmas tree” but thanks to modern apparatus- controllers, jacks etc it is now possible to test the airframe not just rigorously but also precisely using g loads and frequencies collected from actual inflight data rather than a set of tables of loads and durations. This capability now gives us a powerful tool to do a weight improvement routine which can, if properly designed, remove every ounce of excess fat from the airframe. I believe that weight reduction can a change the entire Tejas programme.