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1.3.3. Minimum Control Speed during Approach and Landing: VMCL 

“JAR/FAR 25.149 Minimum control speed 

(f) VMCL, the minimum control speed during approach and landing with all engines operating, is the calibrated airspeed at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the aeroplane with that engine still inoperative, and maintain straight flight with an angle of bank of not more than 5o. VMCL must be established with: 

. The aeroplane in the most critical configuration (or, at the option of the applicant, each configuration) for approach and landing with all engines operating; 

. The most unfavourable centre of gravity; 

. The aeroplane trimmed for approach with all engines operating; 

. The most unfavourable weight, or, at the option of the applicant, as a function of weight. 

. Go-around thrust setting on the operating engines 

(g) For aeroplanes with three or more engines, VMCL-2, the minimum control speed during approach and landing with one critical engine inoperative, is the calibrated airspeed at which, when a second critical engine is suddenly made inoperative, it is possible to maintain control of the aeroplane with both engines still inoperative, and maintain straight flight with an angle of bank of not more than 5 degrees. VMCL-2 must be established with [the same conditions as VMCL, except that]: 

. The aeroplane trimmed for approach with one critical engine inoperative 

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. The thrust on the operating engine(s) necessary to maintain an approach path angle of 3 degrees when one critical engine is inoperative 

. The thrust on the operating engine(s) rapidly changed, immediately after the second critical engine is made inoperative, from the [previous] thrust to: 

- the minimum thrust [and then to] 

- the go-around thrust setting 

(h) In demonstrations of VMCL and VMCL-2, … lateral control must be sufficient to roll the aeroplane from an initial condition of steady straight flight, through an angle of 20 degrees in the direction necessary to initiate a turn away from the inoperative engine(s) in not more than 5 seconds.” 

1.3.4. Minimum Unstick Speed: VMU 

“JAR/FAR 25.107 Take-off speeds 

(d) VMU is the calibrated airspeed at and above which the aeroplane can safely lift off the ground, and continue the take-off…” 

During the flight test demonstration, at a low speed (80 - 100 kt), the pilot pulls the control stick to the limit of the aerodynamic efficiency of the control surfaces. The aircraft accomplishes a slow rotation to an angle of attack at which the maximum lift coefficient is reached, or, for geometrically-limited aircraft, until the tail strikes the runway (the tail is protected by a dragging device). Afterwards, the pitch is maintained until lift-off (Figure B4). 

Two minimum unstick speeds must be determined and validated by flight tests: 

- with all engines operatives : VMU (N) 

-with one engine inoperative : VMU (N-1) 

In the one-engine inoperative case, VMU (N-1) must ensure a safe lateral control to prevent the engine from striking the ground. 

It appears that : VMU (N) ≤ VMU (N-1) 

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1.3.5. Stall Speed 

Air velocity over the wing increases with the angle of attack, so that air pressure decreases and the lift coefficient increases. 

Therefore, the lift coefficient increases with the angle of attack. Flying at a constant level, this lift coefficient increase implies a decrease of the required speed. Indeed, the lift has to balance the aircraft weight, which can be considered as constant at a given time. 

The speed cannot decrease beyond a minimum value. Above a certain angle of attack, the airflow starts to separate from the airfoil (Figure B5). 

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Figure B6 shows that the lift coefficient increases up to a maximum lift coefficient (CLmax), and suddenly decreases when the angle of attack is increased above a certain value. 

This phenomenon is called a stall and two speeds can be identified : 

-VS1g, which corresponds to the maximum lift coefficient (i.e. just before the lift starts decreasing). At that moment, the load factor is still equal to one (JAR 25 reference stall speed).