Interview with pilot and flight instructor Pierre Viby
Pierre Viby is a pilot and flight instructor with experience in flying a Boeing 767. He is associated with Center Air, a center for pilot training in Roskilde, Denmark. 911facts.dk contacted CenterAir3 in April 2013 and was after a conversation with training manager Jes Bisgaard referred to Pierre Viby, who is the center’s most knowledgable person about flying and training in Boeing 767 planes. Pierre Viby most graciously answered all questions by email.
To increase readability, we have in this article divided the correspondance, so that questions will be answered in the order they were posed. The complete correspondance0 can be found in “Sources”.
The editors of 911facts.dk explained to Pierre Viby that many conspiracy theories concerned themselves with the plane that hit the World Trade Center 2 on September 11, 2001. The main claim is that it could not have been Flight 175 as the official investiation concluded.
The conspiracy theorists claim:
- that the plane could not have reached the 942 km/h when hitting the tower. This velocity is measured with a precision of +/- ca. 30 km/h.
- that the plane would disintegrate if the velocity exceeded max cruise speed (mach 0,86, 913 km/h), which is the top velocity for this type as stated by Boeing1.
- that it was not possible for the hijackers to hit the World Trade Center 2 with a velocity of 942 km/h, because it was a small target at that velocity and because the pilot hijacker was a relatively inexperienced pilot especially with that type of plane.
We also informed Pierre Viby that the impact happened at approximately 1,000 feet altitude after the plane had decreased altitude from circa 14,000 feet two minutes before, or about 6,000 feet per minute2.
Pierre Viby gave some technical information:
“A plane’s maximum velocity also called VNE (Velocity Never Exceed) is calculated from a velocity 0.9 times VDive. VDive is given as the velocity where a plane will begin to experience structural damage. It does NOT mean that if the plane hits or exceeds this velocity, it will disintegrate at once but that structure damages will begin to appear. My guess here is that a Boeing 767 as well as other planes will experience too big a load on wings and tail rudder, so this will be where the first damages will be seen.”
Questions and answers
- Is it possible to go to overspeed in a 767-200 without major problems?
“Yes. No problems.”
- Is it possible to reach 942 km/h or more at an altitude of 1,000 feet, e.g. in connection with a dive of 6,000 feet per minute?
“Yes. A dive of 6,000 feet per minute over two minutes while having maximum thrust of the engines would not be a problem. The plane would already begin to accelerate when diving 1,000 to 1,500 feet per minute with the engines running idle.”
- Is it possible to reach 942 km/h or more at an altitude of 1,000 feet if the plane is not in a dive, but merely flies straight ahead, disregarding the rules?
“Perhaps. It would depend somewhat on the engine type, the weight of the plane, wind, weather and temperature. But under the right conditions it is possible.”
- Is it correct that Boeing’s max cruise speed is the recommended maximum speed for the most economical and/or most safe way of flying but is not a technical limitation when it comes to how much engine and plane can perform physically in the extreme?
“Maximum velocity is as explained above a design velocity. All planes will be able to reach higher velocities without problems.”
- Will a Boeing 767-200 disintegrate if flown at 942 km/h at 1,000 feet?
“No, but there would certainly be structural damage at this height.”
- Can it be stated with certainty when a plane will disintegrate due to velocity?
“No, not if disintegrating means that the plane will fall completely apart in the air.”
- Is it correct that flying a 767-200 will become really difficult and dangerous when approaching the speed of sound which is about 1,100-1,200 km/h at 1,000 feet?
“Yes. Not that it is not already dangerous to exceed the plane’s VNE velocity, but when you approach the speed of sound, some aerodynamic changes will occur, which a passenger plane in no way is designed for.”
- Can a relatively untrained pilot or amateur fly a 767-200 into a target like WTC 2 (which was about 65 m wide) with 942 km/h after having taken control while in the air diving circa 6,000 feet a minute in the last couple of minutes?
“Yes, that is unfortunately probably possible.”
- Have you ever tried yourself to fly at overspeed in a 767?
“No, never. But flown up to about VNE velocity without any problems.”
These answers opened up for some in-depth questions:
- So “maximum cruise speed” is the same as VNE and is therefore relative to the speed of sound (that is, altitude and temperature), right?
- And VNA is thus 913 km/h at 35,000 feet which Boeing states here1. That means that VD is 913/90 * 100 = 1,014 km/h at 35,000 feet, right?
- But then VNA must be somewhat higher at 1,000 feet, since the speed of sound is higher there, right?
- As far as I remember, the speed of sound is about 1,100 km/h at 1,000 feet, ergo VNE must be circa 1,100 * 0.9 = circa 990 km/h, right?
- If so, I am wondering how you arrived at the certainty of structural damages at 942 km/h at 1,000 feet. Is that because I have overlooked or misunderstood something?
Pierre Viby replied:
“A big plane’s maximum velocity Vne/Mne (Velocity or Mach number Never Exceed) is indicated both as knots and Mach number. At high altitude it is the plane’s Mach number, for a Boeing 767 0.89 Mach which is the maximum, and for lower altitudes it will hit 395 knots as Vne, before it hits the Mach limitation. It must be said that the velocities can differ a bit, depending on which version of Boeing 767 we are talking about.
It gets a bit technical if we go into detail, but in short Vno/Mmo a lower velocity since it must be considered design-wise that the plane must be able to be maneuvered with a certain assigned safety margin.”
- Flight 175 can have reached the 942 km/h +/- approximately 30 km/h, which it hit the tower with.
- Flight 175 would not disintegrate if it flew faster than max cruise speed (mach 0,86, 913 km/h).
- It was possible for the hijackers to hit the World Trade Center 2 with a velocity of 942 km/h, even though the target was small at such a velocity and the hijacker pilot was a relatively inexperienced pilot also with that type of plane.
Thus, there is nothing that points to other explanations than terrorists from al Qaeda hijacking Flight 175 flying it into the World Trade Center 2.
- Email korrespondance med Pierre Viby, PDF
- Boeing 767, Wikipedia
- “Flight Path Study-United Airlines 175,” Feb. 19, 2002
[Chapter 1, The 9/11 Commission Report, “We Have Some Planes.” Footnote 41]
- Center Air