Comments by "Keit Hammleter" (@keithammleter3824) on "YOU have Misunderstood The De Havilland Comet. It Wasn’t the Windows!" video.

  1.  @phonicwheel933  In your first sentence, you've got it in one. FMEA lets you see safety deficiencies BEFORE they cause accidents. Re ADF windows,, I'm speculating here - since I don't have access to DeHavilland records - but since the ADF antenna and it's electrically insulating surround was supplied to DeHavilland by their usual supplier, it would have been the supplier's responsibility to design it to cope with presurisation and largely the supplier's responsibility to test it against presurisation. But DeHavilland didn't tell them it was for a pressurised aircraft - that was the problem. However, DeHavilland should have at least audited the supplier's records to discover and verify whatever testing was done, given any hull penetration is flight critical. I guess they didn't.
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  2. Post 3: At 21:10, Mentour Now talks about what the aircraft industry owes to the Comet - the good that came out of the Comet disasters. He talked about teaching the industry the need for failsafe systems and robust fatigue testing. Well, the need for failsafe systems was already known, just maybe not in DeHavilland. The formal engineering discipline for achieving fail safe systems in FMEA (Failure Mode Effects Analysis) which was well established in the industry, particularly in the USA. FMEA as a critically important discipline was established during WW2, when aircraft engineers needed to ensure that military aircraft did not crash unless shot at, and that aircraft should survive being shot at to the maximum degree feasible. Metal fatigue was also understood, since all metal aircraft go back to before WW2. Just not well understood within DeHavilland. Here we see the result in poor quality journalism again, originating falsehood just like they did with "square" windows. It was reported early in the Comet crash history that the stress on the Comet skin in places exceeded the limit for the alloy used. However the limit then, which was somewhat of a DeHavilland engineering guesstimate, was actually stricter than later knowledge showed it needed to be. The real benefit of the Comet was two things:- a) it showed the British certification authority that they needed to do more than just rubber-stamp whatever the manufacturers gave them; b) It really shook up British aviation accident investigation authority - showed them forcibly that they needed to lift their game. The purpose of accident/incident investigations is not to lay blame, it is to find a system or process cause so the industry can eliminate that cause so it won't happen again. When a Comet 1 takeoff incident occurred, the Accident Investigation blamed the pilot. When it happened again they again blamed the pilot. Blaming pilots is a cop-out that solves nothing and achieves nothing. Later they realised the incidents occurred due to aerodynamic problems and was not the pilot's fault at all. I have sat on incident inquiry panels myself (not in the aircraft industry). You always ask what could have happened APART from an operator error. Even if you are certain that it was operator error, you have to ask WHY did the operator err? Was it a training deficiency, recruitment failing, instruments misleading, etc. 99/100 there is a reason, and if there isn't, the process tells you what was wrong with the structure or system.
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  3. Post 2: Mr Mentour Now got my attention at 7:07 when he asked why did the Comet 1 have presurisation problems while other aircraft already in service did not. He startled me at 10:01 when he claimed that the higher altitude of the Comet 1 "meant a much higher pressure differential". That's not right. A rule of thumb applies: air pressure halves for each 5000 metres of added altitude. But since it is the pressure difference between inside the cabin and the outside that matters, not the absolute pressure, we should calculate what the difference in precent is, taking the edge of space (zero pressure) as 100%. it is common for aircraft to not be pressurised to sea level pressure - this saves a little bit of weight. Civil aviation rules require a maximum cabin equivalent altitude of 2400 metres, but various aircraft have been designed for 1500 and 2000 metres cabin equivalent altitude. Here is the data for an aircraft pressurised for sea level and for 2000 m:- Altitude metres % difference cabin sea level % difference cabin at 2000 m 0 0% - 5,000 47 31 10,000 (33,000 feet) 75 67 15,000 (49,000 feet) 89 84 20,000 95 94 We see that the airframe stress goes up but certainly not a MUCH greater amount at the Comet's higher altitude - it's a modest approximately 20% more stress. In any case, there was nothing new that the engineers had to figure out or learn to design for that 20% increase - it was merely a matter of doing the established calculations with the correct data. As Mentour Pilot said, pressurised airliners go back 10 years before the Comet. Use a few more rivets, maybe a slightly thicker sheet for the skin. So, the question is: Why did Dehavilland get it wrong? Answer: Because they had no relevant experience in the company. Unlike the other manufacturers, they had no high altitude transport experience - they didn't realise its not sufficient to calculate skin stresses. You have to tell subcontractors things like radio antenna need to be designed for pressurisation too. They didn't. Dehavilland engineers in designing an all-metal high altitude airliner were like a cardiac surgeon doing brain surgery - yeah, he knows the essential basic principles, but he is not likely to get a good result. It has long been established practice in the American aircraft industry - conduct Failure Mode Effects Analysis (FMEA) - this is a formal engineering discipline that ensures any likely failure will not be catastrophic or kill people. Essentially, they look at each component, identify how it can fail, and if it does, what that will lead to. As in "suppose the glue on the radio antenna fails? Oh, it will lead to skin rupture - that will cause hull failure. Right, we better fix that." Clearly, Dehavilland's engineers did not do an FMEA on the Comet 1, or if they did, they took shortcuts
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  4. Post 1: Mentour Now stated that "engine access was said to be satisfactory". All of us engineers and technicians who have worked on British and American equipment know what "satisfactory' means when it is said by a British manufacturer. It means the access is difficult and/or requires the undoing of a stupendous number of bolts and/or disconnection of systems.
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