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Thread: Vmo vs MMo
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Old 27th Jan 2006, 18:49
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Canuckbirdstrike
 
Join Date: Dec 2000
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Hmmm... My understanding is that Vmo can be influenced by bird strike certification standards. The certification standard from FAR 25.571 is "Impact with a 4 lb bird at Vc (design Cruise Speed) at seal level or 0.85Vc at 8,000 feet whichever is more crtitical". This is for a FAR 25 aircraft windshield or airframe, for the empennage the speed is the same but the bird weight is 8 lbs (due to the loss of a Viscount in 1962). The design requirement is "The airplane must be capable of successfully completeing a flight during which likely structural damage occurs" (doesn't say how challenging that successfu landing might be and whether you get to use the airplae again in the near future).

My research shows that Vc is a function of Vd (Design Dive Speed) and Vb (Design Speed for Maximum Gust Intensity). All the details including a diagram are defined in FAR 525.335 with an explanation of the manoeuvring envelope contained in FAR 525.333. Reading in the text there is a reference that states that Vc cannot be greater than 0.8 Vd. Vmo is required to be less than Vd. Based on my understanding, if an aircraft fails the FAR 25.571 test at 280 knots then following the trail through the the FARS would cause Vmo to be limited to some value less than 280 knots. Th net result is that Vmo becomes limited by FAR 25.571 (Bird Strike) or FAR 525.333 & 335 whichever is more limiting.

Just my opinion based on reading very complex inter-linked FARS by a humble pilot.

Richard
A320 Captain and bird strike researcher
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