Mailing List lml@lancaironline.net Message #45514
From: <rwolf99@aol.com>
Sender: <marv@lancaironline.net>
Subject: Reliability Question
Date: Thu, 27 Dec 2007 21:46:38 -0500
To: <lml@lancaironline.net>
This is a question for Brent and Hamid, but it also addresses the recent thread on reliability and redundancy.

I have been recently educated on the FAA approach to reliability for certified airplanes, which can be simply summarized as a connection between the probability of failure and the resulting consequences.  For the aircraft I was working with, a "catastrophic" failure (results in loss of aircraft, loss of all life) needed to have a probability of 10e-9 (that's one in a billion).  A "major hazard" (damage to aircraft, loss of a passenger) needed to have a probability of 10e-8.  Similarly, lower consequences were allowed to happen more frequently.  The key point here is that there was no failure that was prohibited -- even a wing falling off.  We just had to make it so unlikely that the risks were acceptably small.  Acceptable risks are lower for airliners, and higher for two-person propeller airplanes.

FWIW, this goal was accomplished by making systems -- not components -- more reliable by strategically adding redundancy where it improved systm reliability.

My question for Brent and Hamid is this -- given your comments that an EFIS has a finite probability of failure, what is that number?  What would it be for a certified EFIS (like a Chelton) and what would it be for a non-certified system (like a Dynon)?

What I am getting at here is a quantitative way of ensuring that our proposed flight display systems have a reliability equal to that on a certified aircraft.  For example, if a Chelton failure probability is one in a million, and a Dynon failure probability is one in a thousand, and the power supplies are separate (Dynon internal battery backup), wouldn't a Chelton + Dynon give you a one in a billion probability of total failure?

By the way, I personally don't believe the failure probability numbers that the systems safety engineers come up with.  I believe that the actual rate of failure is significantly higher.  However, I *do* believe that using this methodology has given us a fleet of aircraft whose reliability we are comfortable with.

- Rob Wolf

More new features than ever. Check out the new AOL Mail!
Subscribe (FEED) Subscribe (DIGEST) Subscribe (INDEX) Unsubscribe Mail to Listmaster