SomebodyInTLS wrote:frmrCapCadet wrote:https://www.discovermagazine.com/the-sciences/how-to-avoid-repeating-the-debacle-that-was-the-space-shuttle
This is a somewhat similar sort of disaster. There may be a better NYT article but it is behind a more formidable paywall. You likely could get it through any major library system.
I as disturbed by a statement that a plane is safe/not safe and that it is binary. Much prefer a statistical/probability although that can be compatible with 'binary'.
Me too. It's actually frightening that Pythagoras believes in safe/not-safe like that!
Let me give you an examples of what I mean.
In 1962, a United Airlines Vickers Viscount N7430 struck a flock of whistling swans at an altitude of 6000 feet. One of the swans impacted the horizontal stabilizer which penetrated the leading edge and subsequently damaged the elevator which led to a loss of control of the airplane. See summary here:
https://aviation-safety.net/database/record.php?id=19621123-1.
As a result of this accident, the regulations were amended in 1970 as follows:
§ 25.631 Bird strike damage.
The empennage structure must be designed to assure capability of continued safe flight and landing of the airplane after impact with an 8-pound bird when the velocity of the airplane (relative to the bird along the airplane's flight path) is equal to VC at sea level, selected under § 25.335(a). Compliance with this section by provision of redundant structure and protected location of control system elements or protective devices such as splitter plates or energy absorbing material is acceptable. Where compliance is shown by analysis, tests, or both, use of data on airplanes having similar structural design is acceptable.
[Amdt. 25-23, 35 FR 5674, Apr. 8, 1970]
Forensic investigation revealed that the whistling swan that struck the Viscount had a weight of 12 lbs. Canada geese typically have a weight between 8 and 12 lbs. For the most part, Canada geese migrate at low altitudes, below 3000 feet, but have been reported at altitudes as high as 29,000 ft. On 29 November 1973, a Riippell’s Griffon, which can weigh between 14 to 20 lbs, collided with a commercial aircraft at 37,000 ft over Abijan, Ivory Coast. The altitude is that recorded by the pilot shortly after the impact, which damaged one of the aircraft’s engines and caused it to be shut down. Another example is the Andean condor which has a weight of over 30 lbs and the ability to fly up to altitudes of 16000 ft.
FAA and EASA (JAA) regulations, under which airplanes are certified, were developed using analysis completed specific for airline operations within North America and Europe, which presume a threat environment and exposure subject to these locales. Yet, we would still say that an airplane designed to an 8 lb bird impact would be safe considering the increased weights and behavioral characteristics of birds in South America, Africa and the Asian sub-continent.
From a statistical basis, it is a mathematical impossibility to develop probability curves from observational data to characterize what a 10E-9 per flight hour event is for birdstrike . There just have not been enough flights, especially if one were to expand the analysis to Africa and Asian sub-continent.
If one actually reviews the record of how the 8 lb rule was developed, it becomes one of sausage-making as the FAA essentially just picked 8 lbs as a number that was good enough.
Furthermore, there are inconsistencies as FAR 25.571 prescribes a 4 lb bird for other structure and 25.775 prescribes a 4 lb bird for windshield panes. Recognizing these inconsistencies, the FAA in 1993 undertook an action to update and harmonize the regulations for birdstrike:
Task 1-Bird Strike Damage: Develop new or revised requirements for the evaluation of transport category airplane structure for in-flight colilision with a bird, including the size of the bird and the location of the impact on the airplane (FAR 25.571, 25.631, 25.775 and other conforming changes).
[Federal Register Vol.58, No. 48, Monday March 15,1993, pg 13817]
After 10 years of study, the FAA and EASA concluded that it was not possible to reconcile the differences and left the regulations unchanged.
I could point to other examples, like 16g seats and 90 seconds for emergency evacuation. A seat designed to 16g is a "safe" seat. An airplane that can be evacuated in 90 seconds is a "safe" airplane. The point here though is that the industry--OEM, operators and regulators--came to a consensus as to what an acceptable level of safety is. And the airplane manufacturer is obligated to meet that requirement and nothing more to provide a safe airplane.
The level of consensus though is subject to change over time as new information and new technology is developed. Furthermore, I would acknowledge that the regulations are not all encompassing and a manufacturer cannot put his head in the sand and hide behind regulations to avoid taking action. That of course is a given.
I don't want you to go away with the impression that I absolve Boeing of designing a flawed MCAS system. Far from it. My point is that the process of certification is what makes an airplane safe and in the case of the 737Max the process was woefully flawed.