Vd Dive Speed -- Overspeed Protection?
Thread Starter
Join Date: Aug 2014
Location: Miami, FL
Posts: 17
Likes: 0
Received 0 Likes
on
0 Posts
Vd Dive Speed -- Overspeed Protection?
I'm hoping someone could shed more light on how the FAA handles overspeed/autothrottle protection (e.g., wrt FAR Part § 23.335) in determining Vd (design dive speed).
As I read these regs, and I'm no lawyer (thank God), Section 335(b)(4) is the one manufacturers shoot to use in determining Vd/Md, because it gives the most flexibility. And within this subsection, 335(b)(4)(i) is the paragraph I'm really curious about.
Within this subsection 335(b)(4), paragraphs (ii) and (iii) set a cap on the maximum Mach # for the test if above the transition altitude, so determining Md is easy to understand (e.g., 0.07 Mach above Mmo for commuter aircraft). But Vd in paragraph (i) seems to basically be about the highest speed the plane reaches in a prescribed dive and pullup maneuver while being required to maintain cruise power.
And therein lies my question -- what about aircraft with autothrottle/envelope protection to retard the throttles and prevent speeds from becoming excessive during just such a maneuver? Would the FAA allow such a system to impose a reasonable limitation of the diving speed during this maneuver (say to prevent flutter) -- or would they require the autothrottle/envelope protection be disabled for the test?
As I read these regs, and I'm no lawyer (thank God), Section 335(b)(4) is the one manufacturers shoot to use in determining Vd/Md, because it gives the most flexibility. And within this subsection, 335(b)(4)(i) is the paragraph I'm really curious about.
Within this subsection 335(b)(4), paragraphs (ii) and (iii) set a cap on the maximum Mach # for the test if above the transition altitude, so determining Md is easy to understand (e.g., 0.07 Mach above Mmo for commuter aircraft). But Vd in paragraph (i) seems to basically be about the highest speed the plane reaches in a prescribed dive and pullup maneuver while being required to maintain cruise power.
And therein lies my question -- what about aircraft with autothrottle/envelope protection to retard the throttles and prevent speeds from becoming excessive during just such a maneuver? Would the FAA allow such a system to impose a reasonable limitation of the diving speed during this maneuver (say to prevent flutter) -- or would they require the autothrottle/envelope protection be disabled for the test?
Last edited by Mach Stall; 8th Nov 2015 at 09:28. Reason: clarity
Join Date: Jun 2006
Location: Moses Lake, WA
Age: 63
Posts: 53
Likes: 0
Received 0 Likes
on
0 Posts
It would come down to the reliability of the protective function, whether it can be overridden by the pilots and whether the manufacturer wishes to have dispatch allowed with that function inoperative.
For autothrottle, every manufacturer would want the MMEL to allow dispatch with autothrottle inoperative, with no requirement to reduce the maximum allowed speed, so I would be very surprised if they would propose to take credit for autothrottle to reduce thrust during this test.
FBW systems often include overspeed protection functions that cannot be overridden by the pilots, and these systems are operative during these flight tests. The pilot must push the control further and further ahead as the aircraft accelerates during the dive, and eventually ends up with full forward control and the airspeed or mach stabilized at the limit set by the automatic high speed protection.
For autothrottle, every manufacturer would want the MMEL to allow dispatch with autothrottle inoperative, with no requirement to reduce the maximum allowed speed, so I would be very surprised if they would propose to take credit for autothrottle to reduce thrust during this test.
FBW systems often include overspeed protection functions that cannot be overridden by the pilots, and these systems are operative during these flight tests. The pilot must push the control further and further ahead as the aircraft accelerates during the dive, and eventually ends up with full forward control and the airspeed or mach stabilized at the limit set by the automatic high speed protection.
In the days before you tube Boeing tested the 747 to .99 Mach.
Not just in the days before youtube - the 747-8 was taken to 0.98+ Mach as well during flutter testing.
