Balsamo Busted [View all]
Rob Balsamo, founder of Pilots for 9/11 "Truth", is selling a video which claims that the 757s and 767s hijacked on 9/11 were clocked at "impossible" speeds, beyond their Design Dive Velocities (Vd/Md). The video claims that the high speeds imply that the planes must not have been standard 757s and 767s, implying of course, an "inside job" conspiracy.
I posted a comment noting that the claim was nonsense because airplanes are designed to be safe at their Vd speeds, and the way the engineers would assure that is by adding a 50% margin of safety to their calculated stresses at that speed. That's because the "real-world" stresses and structural strength can only be approximated. This is a basic principle of structural engineering. Long story short, over the course of far too many posts, Balsamo claimed that's not true, that the only margin of safety for speed was between the Maximum Operating Velocity (Vmo/Mmo) and Vd. As his one and only reference for this claim, Balsamo further claims that he is "certified by the FAA to teach this material."
To settle the issue (with a source that Balsamo can't refute or denigrate), I sent the following question to the FAA:
limit loads (the maximum loads to be expected in service) and ultimate
loads (limit loads multiplied by prescribed factors of safety). Unless
otherwise provided, prescribed loads are limit loads."
FAR 25.303 says, "Unless otherwise specified, a factor of safety of 1.5
must be applied to the prescribed limit load which are considered
external loads on the structure."
FAR 25.305(e) says, "The airplane must be designed to withstand any
vibration and buffeting that might occur in any likely operating
condition up to VD/MD, including stall and probable inadvertent
excursions beyond the boundaries of the buffet onset envelope."
I don't need a great deal of detail, but my question is: For stresses
caused by velocity, is there a margin of safety beyond Vd/Md (i.e. are
those considered to be limit loads or ultimate)?
Today, I received the following response (with the relevant parts emphasised by me):
From Gregory.Johnson@faa.gov
To ws@#######.###
Date Today 13:58
Mr. Seger,
This correspondence is in response to your inquiry submitted to the Denver Flight Standards Office relevant to design loads. As you correctly identified in your review of 14CFR part 25, structural design loads are are identified in terms of Ultimate (Ult) and Limit loads.
Limit loads are the maximum load that it is anticipated the structure will see in service. By rule the aircraft structure must be capable of sustaining Limit loads without permanent detrimental deformation. For all loads up to Limit, deformation of the structure must not interfere with the safe operation of the aircraft. Thus consider the limit load as the operational limit.
Design loads are referred to a Ultimate loads which are bound by material properties F = P/A
Ultimate load = Limit load x a factor of safety (1.5) thus Ult. = Limit x 1.5
In design it must be assumed that while the airframe is not intended to experience loads in excess of Limit, a margin or reserve capacity is necessary to preclude structural failure thus the 1.5 factor as the boundary to define Ultimate. As the material capacity is a constant, the design (section area) is the variable to react to the applied loads and preclude structural failure. The loads then that are considered would include such loads flight maneuver, gust, torsion, delta P. From a basic loads stand point an interaction equation would be applied to address these loads. This 1.5 margin then defines the limit within which the aircraft may be assumed to safe operate within the parameters of these type of loading conditions.
An airframe is certified to a maximum velocity that is can sustain in flight and a maximum acceleration to which it can be subjected to and sustain safe flight, maneuver and landing. Each of these criteria are defined in 14CFR under Subpart C. The maximum velocity and acceleration as a design criteria are potentially the most significant considerations of a design for operation within the defined Limit load.
I hope this addresses your question
Regards.
Gregory Johnson
Denver Aircraft Certification Office
ANM100D
Phn: (303) 342-1083
E-mail: gregory.johnson@FAA.gov
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Game over.
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