Airspeed Limitations

[BeechBoy]

The airspeed limitations on the Dornier 328 are 270 KCAS from SL-8,000 feet, gradually increasing to 300 KCAS at 10,000 MSL.

Why can you go 300 KCAS at 10,000 MSL but are limited to 270 KCAS below 8,000? If the airplane can withstand the dynamic pressure and parasite drag at 10,000 why not at 8,000? This makes absolutely no sense to me .

 

[tgrayson]

The airspeed limitations on the Dornier 328 are 270 KCAS from SL-8,000 feet, gradually increasing to 300 KCAS at 10,000 MSL.

Why can you go 300 KCAS at 10,000 MSL but are limited to 270 KCAS below 8,000? If the airplane can withstand the dynamic pressure and parasite drag at 10,000 why not at 8,000? This makes absolutely no sense to me .

The Type Certificate Data Sheet for this aircraft merely shows Vmo as 270 KIAS. Are you certain that 300 KCAS is denoted as Vmo?

Anyway, speculation:

Dynamic pressure isn't always the cause of airspeed limitations. You also have flutter, mach effects, and the ability to withstand gusts of certain intensity without exceeding the load factor limit.

Part 25 allows the gust intensity to be scaled back with altitude, which would allow faster airspeeds, so this is consistent with your data.

 

[BeechBoy]

The Type Certificate Data Sheet for this aircraft merely shows Vmo as 270 KIAS. Are you certain that 300 KCAS is denoted as Vmo?

Anyway, speculation:

Dynamic pressure isn't always the cause of airspeed limitations. You also have flutter, mach effects, and the ability to withstand gusts of certain intensity without exceeding the load factor limit.

Part 25 allows the gust intensity to be scaled back with altitude, which would allow faster airspeeds, so this is consistent with your data.

I'm sure about 300 KCAS being defined as Vmo. It's on a graph in our AFM.

I would imagine that Mach effects (including flutter) would increase as altitude increases and would result in a lower Vmo at higher altitudes.

I never heard of the gust intensity limitation you referred to.

As I type this I remember something that sounds so ridiculous I could never have made it up. At one time I heard something about the windshield not being able to withstand a birdstrike above 270 KCAS. Since the chance of a bird strike is greater closer to the ground Vmo was limited to 270. (Honest guys, I have NOT been drinking ).

 

[NJA_Capt]

Why can you go 300 KCAS at 10,000 MSL but are limited to 270 KCAS below 8,000?This makes absolutely no sense to me .

In the case of some airplanes, it is a bird strike criteria. It has nothing to do with aerodynamics.

Besides, with a 250 limit below 10K, 300 kts does nothing for you anyway.

Our plane is limited to 270 below 8000 also.

As a matter of fact, here is an excerpt from the Citation X type certificate. These exemptions are listed on the aircraft registration also.


Exemptions: (a) No. 6179, exemption from bird impact requirements of § 25.571(e)(1);
http://www.airweb.faa.gov/Regulator...0c1316beed862572430068209f/$FILE/T00007wi.pdf

FAR 25.571 (e)(1)
(1) Impact with a 4-pound bird when the velocity of the airplane relative to the bird along the airplane's flight path is equal to Vcat sea level or 0.85Vcat 8,000 feet, whichever is more critical;
Electronic Code of Federal Regulations:

 

[tgrayson]

I would imagine that Mach effects (including flutter) would increase as altitude increases and would result in a lower Vmo at higher altitudes.

I agree.

I never heard of the gust intensity limitation you referred to.

Not surprising...buried in Parts 25 and 23. No reason the pilot should know.

As I type this I remember something that sounds so ridiculous I could never have made it up. At one time I heard something about the windshield not being able to withstand a birdstrike above 270 KCAS. Since the chance of a bird strike is greater closer to the ground Vmo was limited to 270. (Honest guys, I have NOT been drinking ).

Looks like only the sea level velocity is important in this area:

§ 25.775 Windshields and windows.
(a) Internal panes must be made of nonsplintering material.
(b) Windshield panes directly in front of the pilots in the normal conduct of their duties, and the supporting structures for these panes, must withstand, without penetration, the impact of a four pound bird when the velocity of the airplane (relative to the bird along the airplane's flight path) is equal to the value of VC, at sea level, selected under § 25.335(a).
(c) Unless it can be shown by analysis or tests that the probability of occurrence of a critical windshield fragmentation condition is of a low order, the airplane must have a means to minimize the danger to the pilots from flying windshield fragments due to bird impact. This must be shown for each transparent pane in the cockpit that -

 

[ATPInstructor]

Right on guys, low airspeed numbers at low altitudes are often bird strike based.

Also, concerning low altitude numbers above 250, they can be helpful. When I train JAA clients from Europe they state they often get approved for much higher then 250 below 10,000 there.

Remember it is much different there, i.e. they have low transition levels and often rotate to 29.92 at 5-6 thousand feet.

Bill

 

[tgrayson]

In the case of some airplanes, it is a bird strike criteria. It has nothing to do with aerodynamics.

There is a reference to the magic altitude of 8,000 in 25.521:
----------------------------
(e) Damage tolerance (discrete source) evaluation. The airplane must be capable of successfully completing a flight during which likely structural damage occurs as a result of -
(1) Impact with a 4-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 or 0.85 Vc at 8,000 feet, whichever is more critical;

----------------


So that is circumstantial evidence that it is bird-strike related, although not the windscreen.

 

[tgrayson]

The airspeed limitations on the Dornier 328 are 270 KCAS from SL-8,000 feet, gradually increasing to 300 KCAS at 10,000 MSL.

One thing that doesn't jibe with this being a birdstrke is the "gradually" increasing airspeed past 8,000. There is no provision in Part 25 for taking into account a gradually increasing probability of a bird strike. Based on the section I quoted, the Vmo should have jumped to 300 KCAS.