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Old 29th Jan 2013, 05:48   #1 (permalink)
 
Join Date: May 2010
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Questions about vmcg

1,The critical engine for vmcg
The ac25-7 suggests "the critical engine for ground minimum control speed testing should be determined during the takeoff ground run using techniques similar to these described in paragraph 23b(1)." I understand how to determine the critical in the air, but how to do on the ground, is it nessassery for the twin-turbofan engine airplane?
2, How long should be ok between cuting the critical engine fuel and the pilot finishing the action of recovery.
Some books suggest the total minimum allowable intervention time should be the time for aircraft to achieve change of rate about any aixs of 3 deg/sec or the time to reach a change of "G" in any axis of 0.2 or time for an attention getter to function plus the pilot reponse time which is 0.5s. Some books suggest the time should not be less than 1s. Is there some suggestions from experience or statistics.
3, How to set the longitudinal trim and control.
Because nose wheel friction affects the Vmcg, the longitudinal trim should be trimmed nose up, am i right? When the Vmcg is tested, the pilot may push the stick to keep the airplane on the runway preventing the nose up. Too much pushing will increase the friction to decrease the Vmcg. How much pushing is suitable for the pilot, and should the Vmcg be caculated with the different friction.

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Old 29th Jan 2013, 11:04   #2 (permalink)

 
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When the Vmcg is tested, the pilot may push the stick to keep the airplane on the runway preventing the nose up.

No. When the manufacturer establishes VMCG, it is done with nosewheel steering disconnected.

This is to ensure that adverse runway conditions are taken into consideration - ie the worst scenario.
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Old 30th Jan 2013, 22:55   #3 (permalink)
 
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[QUOTE="Lightning Mate;7662920"]When the Vmcg is tested, the pilot may push the stick to keep the airplane on the runway preventing the nose up.

No. When the manufacturer establishes VMCG, it is done with nosewheel steering disconnected.

This is to ensure that adverse runway conditions are taken into consideration - ie the worst scenario.[/QUOTE
yes to mate,but the airplane is set to after CG and nose up trim.if pilot not to push ,the aircraft may be nose up
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Old 24th Feb 2013, 22:09   #4 (permalink)
 
Join Date: Sep 2002
Location: La Belle Province
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Quote:
Originally Posted by fanxiyi2003 View Post
1,The critical engine for vmcg
The ac25-7 suggests "the critical engine for ground minimum control speed testing should be determined during the takeoff ground run using techniques similar to these described in paragraph 23b(1)." I understand how to determine the critical in the air, but how to do on the ground, is it nessassery for the twin-turbofan engine airplane?
2, How long should be ok between cuting the critical engine fuel and the pilot finishing the action of recovery.
Some books suggest the total minimum allowable intervention time should be the time for aircraft to achieve change of rate about any aixs of 3 deg/sec or the time to reach a change of "G" in any axis of 0.2 or time for an attention getter to function plus the pilot reponse time which is 0.5s. Some books suggest the time should not be less than 1s. Is there some suggestions from experience or statistics.
3, How to set the longitudinal trim and control.
Because nose wheel friction affects the Vmcg, the longitudinal trim should be trimmed nose up, am i right? When the Vmcg is tested, the pilot may push the stick to keep the airplane on the runway preventing the nose up. Too much pushing will increase the friction to decrease the Vmcg. How much pushing is suitable for the pilot, and should the Vmcg be caculated with the different friction.
1. Yes, it is necessary to determine the critical engine for VMCG, otherwise you may not be testing the most demanding case, and thus would have an unsafe margin if the actually critical engine were to fail.
What is being suggested in the AC is to alternate setting max power on one engine and idle on the other (which is what 23b(1) states:
Quote:
Originally Posted by AC25-7C 23b(1)(a)
By alternating power or thrust on/power or thrust off from left to right, the critical engine can be defined as the idle engine that requires the highest minimum speed to maintain a constant heading with full rudder deflection.
The difference is that for VMCG it is suggested this be done "during the takeoff ground run".
It may of course be possible to dispense with this step and determine the critical engine analytically (or determine that no engine is critical by design) in which case basic airframe asymmetry and trim requirements will dominate.

2. There is certainly no minimum required reaction time for VMCG tests. While not permissible to cheat or anticipate the engine failure, once the pilot has identified the engine failure as per the AC guidance:
Quote:
Originally Posted by AC25-7C 23b(3)(c)
During determination of VMCG, engine failure recognition should be provided by:
1 The pilot feeling a distinct change in the directional tracking characteristics of the airplane; or
2 The pilot seeing a directional divergence of the airplane with respect to the view outside the airplane.
then he should be reacting, frankly, as quickly as possible. VMCG tests are NOT a handling assessment - they are a specific test manoeuvre, used to define a speed which is then used for the purpose of scheduling operational speeds. You want the very best the pilot can give you from the tests. The no "average pilot" behaviour wanted here.

3. Nosewheel friction does not affect VMCG, because VMCG is tested with NWS off and the pilot is permitted only to use the rudder for directional control. The Ac actually addresses nosewheel loading:
Quote:
Originally Posted by AC25-7C 23b(3)(f)
VMCG testing should be conducted at aft c.g. and with the nose wheel free to caster, to minimize the stabilizing effect of the nose gear. If the nose wheel does not caster freely, the test may be conducted with enough nose up elevator applied to lift the nose wheel off the runway.
(There is an exception to the NWS off rule, which is for aircraft with a very old certification basis, as follows:
Quote:
Originally Posted by AC25-7C 23b(3)(h)
For airplanes with certification bases prior to Amendment 25-42, VMCG values may be demonstrated with nose wheel rudder pedal steering operative for dispatch on wet runways. The test should be conducted on an actual wet, smooth (i.e., not grooved or PFC) runway. The test(s) should include engine failure at or near a minimum VEF associated with minimum VR to demonstrate adequate controllability during rotation, liftoff, and the initial climbout. The VMCG values obtained by this method are applicable for wet or dry runways only, not for icy runways.
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Old 25th Feb 2013, 03:03   #5 (permalink)
 
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MfS

Your post raises a couple of questions:

Is there ever a determination of the effects of reduced friction on Vmcg? It seems like there might be based on the wet case.

Is there a determination of the effects of crosswind? I know it is not required by the AC, but is it done the OEM's engineering data?

Lastly, while 30' deviation from centerline is the standard, are there tests which address lesser deviations, that is higher speeds for Vef? Perhaps for narrow runways ops.

GF

Yes, I went over on number of questions, please forgive.

Last edited by galaxy flyer; 25th Feb 2013 at 03:04.
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Old 18th Mar 2013, 22:20   #6 (permalink)
 
Join Date: Sep 2002
Location: La Belle Province
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Quote:
Originally Posted by galaxy flyer View Post
MfS

Your post raises a couple of questions:

Is there ever a determination of the effects of reduced friction on Vmcg? It seems like there might be based on the wet case.

Is there a determination of the effects of crosswind? I know it is not required by the AC, but is it done the OEM's engineering data?

Lastly, while 30' deviation from centerline is the standard, are there tests which address lesser deviations, that is higher speeds for Vef? Perhaps for narrow runways ops.

GF

Yes, I went over on number of questions, please forgive.
While I'm waiting for AC to get us a new plane (old one had a fuel system issue it seems. AC has previous with that out of Montreal, I'm happy waiting for a new ride ....)

1. reduced friction on vmcg. AFAIK there's nothing specified though there no doubt is research around. My first wag is that its a secondary effect - the lower sideforce capacity from the mains likely makes the ac deviate less for a given yaw AND come back more slowly.

a bigger issue is that wet v1 is usually set as low as you dare, so a near-vmcg abort is more likely....

2. xwind effect is known for older types where it was in the regs for some. since the regs standardized on no xwind that data is not gathered any more. I seem to recall numbers of the order of 1:1 in terms of delta vmcg vs xwind. I think the nprm etc from when it was dropped discusses this. (not got to hand here)

3. yes indeed. theres guidance material from TCCA at least (AC525-014 IIRC) which sets out how to derive the deviation needed for a given runway width and gear geometry. Its just geometry and maths, so i'm not giving any secrets to say that a certain regional jet not built in spanish works out as about 80ft width for a 30ft deviation, dropping 1ft in deviation per 2 ft in width below 80ft.

once you get to a "narrow" case you also look at other factors than vmcg ... xwind landing being one, for example.
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Old 21st Mar 2013, 12:53   #7 (permalink)
 
Join Date: Mar 2013
Location: Cheshire, UK
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Just a few points to add to the excellent detail above. Many years ago when measuring VMCG on a 4 engined regional jet, we took account of tyre pressure and if I recall, the low pressure tyres which were a customer option had a lower VMCG value.

We also never disconnected the nosewheel steering. Although not used during the tests, it was thought that you might actually need it to avoid deviating on to the grass!

I also think that we published a higher VMCG for narrower runways.

While certificating a two engined business jet formerly built in North Wales, the authorities raised an eyebrow at the pilot reaction times. The almost instanteous reaction times were due to the fact that the first indication the pilot got of engine failure was the movement of the rudder pedals due to the rudder bias system.
I think we may have scheduled VMCG against crosswind when certificating to BCAR. (I might be wrong about that - it was a long time ago)
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