Originally Posted by
fanxiyi2003
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:
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:
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:
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:
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.