CS 25.149 Minimum control speed

1 The determination of the minimum control speed, VMC, and the variation of VMC with available thrust, may be made primarily by means of ‘static’ testing, in which the speed of the aeroplane is slowly reduced, with the thrust asymmetry already established, until the speed is reached at which straight flight can no longer be maintained. A small number of ‘dynamic’ tests, in which sudden failure of the critical engine is simulated, should be made in order to check that the VMCs determined by the static method are valid. 2 When minimum control speed data are expanded for the determination of minimum control speeds (including VMC, VMCG and VMCL) for all ambient conditions, these speeds should be based on the maximum values of thrust which can reasonably be expected from a production engine in service. The minimum control speeds should not be based on specification thrust, since this thrust represents the minimum thrust as guaranteed by the manufacturer, and the resulting speeds would be unconservative for most cases.

During determination of VMCG, engine failure recognition should be provided by: a. The pilot feeling a distinct change in the directional tracking characteristics of the aeroplane, or b. The pilot seeing a directional divergence of the aeroplane with respect to the view outside the aeroplane.

(a)CS 25.149(f) is intended to ensure that the aeroplane is safely controllable following an engine failure during an all-engines-operating approach and landing. From a controllability standpoint, the most critical case usually consists of an engine failing after the power or thrust has been increased to perform a go-around from an all-engines-operating approach.

(b)To determine VMCL, the flap and trim settings should be appropriate to the approach and landing configurations, the power or thrust on the operating engine(s) should be set to the go-around power or thrust setting, and compliance with all the VMCL requirements of CS 25.149(f) and (h) must be demonstrated.

(c)At the option of the applicant, a one-engine-inoperative landing minimum control speed, VMCL (1 out), may be determined in the conditions appropriate to an approach and landing with one engine having failed before the start of the approach. In this case, only those configurations recommended for use during an approach and landing with one engine inoperative need be considered. The propeller of the inoperative engine, if applicable, may be feathered throughout. The resulting value of VMCL (1 out) may be used in determining the recommended procedures and speeds for a one-engine-inoperative approach and landing.

[Amdt 25/26]

(a)For aeroplanes with three or more engines, VMCL-2 is the minimum speed for maintaining safe control during the power or thrust changes that are likely to be made following the failure of a second critical engine during an approach initiated with one engine inoperative.

(b)In accordance with CS 25.149(g)(5) for propeller-driven aeroplanes, the propeller of the engine that is inoperative at the beginning of the approach may be in the feathered position. The propeller of the more critical engine must be in the position it automatically assumes following an engine failure.

(c)Tests should be conducted using either the most critical approved one-engine-inoperative approach or landing configuration (usually the minimum flap deflection), or at the option of the applicant, each of the approved one-engine-inoperative approach and landing configurations. The following demonstrations should be conducted to determine VMCL-2:

(1)With the power or thrust on the operating engines set to maintain a -3 ° glideslope with one critical engine inoperative, the second critical engine is made inoperative and the remaining operating engine(s) are advanced to the go-around power or thrust setting. The VMCL-2 speed is established with the flap and trim settings appropriate to the approach and landing configurations, the power or thrust on the operating engine(s) set to the go-around power or thrust setting, and compliance with all the VMCL-2 requirements of CS 25.149(g) and (h) must be demonstrated.

(2)With the power or thrust on the operating engines set to maintain a -3 ° glideslope, with one critical engine inoperative:

(i)Set the airspeed at the value determined in paragraph (c)(1) above and, with a zero bank angle, maintain a constant heading using trim to reduce the control force to zero. If full trim is insufficient to reduce the control force to zero, full trim should be used, plus control deflection as required; and

(ii)Make the second critical engine inoperative and retard the remaining operating engine(s) to minimum available power or thrust without changing the directional trim. The VMCL-2 determined in paragraph (c)(1) is acceptable if a constant heading can be maintained without exceeding a 5 ° bank angle and the limiting conditions of CS 25.149(h).

(iii)Starting from a steady straight flight condition, demonstrate that sufficient lateral control is available at VMCL-2 to roll the aeroplane through an angle of 20 ° in the direction necessary to initiate a turn away from the inoperative engines in not more than five seconds. This manoeuvre may be flown in a bank-to-bank roll through a wings-level attitude.

(d)At the option of the applicant, a two-engine-inoperative landing minimum control speed, VMCL-2 (2 out) may be determined in the conditions appropriate to an approach and landing with two engines having failed before the start of the approach. In this case, only those configurations recommended for use during an approach and landing with two engines inoperative need be considered. The propellers of the inoperative engines, if applicable, may be feathered throughout. The values of VMCL-2 or VMCL-2 (2 out) should be used as guidance in determining the recommended procedures and speeds for a two-engine inoperative approach and landing.

[Amdt 25/26]

The 20° lateral control demonstration manoeuvre may be flown as a bank-to-bank roll through wings level.

Where an autofeather or other drag limiting system is installed and will be operative at approach power settings, its operation may be assumed in determining the propeller position achieved when the engine fails. Where automatic feathering is not available the effects of subsequent movements of the engine and propeller controls should be considered, including fully closing the power lever of the failed engine in conjunction with maintaining the go-around power setting on the operating engine(s).