How Safe is Too Safe?

December 21, 2018

A version of this article first appeared in the November 2018 edition of our free newsletter, to subscribe click here

We have technical input into many aircraft development programs that are flying or intending to fly prototypes of new aircraft or major modifications of existing aircraft.

These aircraft fly under experimental flight certificates and as such the development team have some leeway when it comes to the standards that should be met to demonstrate safety of flight.

Of course, everyone is working towards aircraft that safeguards the life and well being of the pilot and the life and wellbeing of people on the ground.
However, there is a trade off between safety and cost – and the budget is always limited.

So how do you spend your budget to achieve an adequate level of safety? What is an adequate level of safety? What level of documentation is appropriate?

Initial flights of new or modified aircraft always carry an elevated risk. This is why you use a professional test pilot who is critical in helping manage those risks. But how much risk is appropriate? How do assess the risks? How do you mitigate the risks? What about the risks you don’t know about?

Aspects of a program that affect the level of Risk:

    • The uniqueness of the modification – the unique aspects of the configuration of the aircraft or modification?
      • How similar is your design/configuration to other aircraft. Are you developing a multi propeller, canard, Quadraplane?
    • The maturity of the technology utilized
      • Are you using a new design of engine? Trim servo motors? Glass cockpit? New design of Seat? New composite material? New anything?
    • The complexity of the technology utilized
      • Chaining together tried and tested technology is only as reliable as the compounded reliability of the individual components. How carefully have you looked at the reliability of the aircraft critical systems?
    • The experience and competence of the team
      • Aviation history is full of very smart people doing very dumb things. Intelligence is not a guarantee nor an excuse nor is it a substitute for competence.
    • The maturity of the infrastructure and company systems the team is working within
      • Are you working within a set of company systems that guide you through the process of product development and flight safety?
      • Are you developing these systems as you go along?
      • Have you missed anything?
      • How would you know?
    • The experience and competence of the pilot
      • The test pilot is the final gatekeeper of flight safety risk in the input they give at the flight safety reviews, during high speed taxi and flight. They are responsible for making real time risk assessments – real life and death decisions. You need the best possible pilot and you have to give them the best possible information.
    • The reliability of the predictive tools used by the development team
      • What kind of analytical assessments are flight safety based on? Estimates? Empirical analysis? Software simulation? Lab or ground testing? Did you fly a scale model? How does the likely accuracy of the results of the design process affect safety of flight?
      • The flight envelope of the aircraft
  • How do you plan to conduct your flight testing?
    • What test points are you planning to fly?
    • How cautious are you with regard to expectations for first flight and how aggressive are your plans for envelope expansion?
    • How will you communicate the appropriate level of concern to the pilot?
    • How will this level of risk be reflected in the flight test program?
    • Even the best test pilots get into a warm and fuzzy state as their confidence increases with each successful test flight. Are the risks being reiterated on a regular basis? Is the pilot being reminded of the margins of safety of the flight envelope?

I have just spent a week with a well known part 23 OEM and their approach to flying prototypes reminded me of the difference between composite and metal aircraft. This informs the level of confidence in the structure of each.

Composite aircraft are thought to be sized to withstand ultimate load with maximum undetectable damage at 180F when saturated with humidity. In reality the structure is sized to conservative strain limits that prohibit damage growth at maximum barely visible damage at the hot wet condition and so the ultimate strength of the pristine structure at nominal service conditions usually has very high residual strength (buckling notwithstanding).

Metal aircraft are typically sized to real static failure modes at ultimate level – you have the forgiveness of yielding and plastic redistribution but in general metal aircraft have lower overall static residual strength than composite aircraft at the start of their service life.

Note that the systems are generally the same for both metal and composite aircraft and require the same level of caution and anxiety…..

With composite structure there is always the specter of process control, especially of bonded structure. While this field is now relatively mature it is still the victim of a far greater sensitivity and variability than metal sub assembly processes. Careful process development backed up by some testing and simple shop guidelines have a far greater effect on the final aircraft strength than tweaking the material selection or increasing the bond width by 10%.

To Summarize:

  • Engineers should not forget that there can be large differences between the CAD model & analysis and the actual aircraft that is built and flown. Not only does the engineering have to be rational and correct but the build process also has to be rational and correct.
  • Many incidents occur because of simple errors in the implementation of a good system design or insufficient pre-flight checking and testing of those simple systems.
  • You need a team of people and everyone needs to keep on top of their game.

And finally:

  • You need leadership capable of understanding all the issues and making the most difficult decision of all: Not to fly.

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