Single engine twins
Guest
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I'm getting the vibe - from the two postings - that watford agrees with the 5 degrees stuff IF PERFORMANCE IS CRITICAL - but suggests that if not (say descending on an instrument approach at a high enough IAS - where downward movement is to be encouraged) it is better to be wings level.
I don't think anybody is arguing that it should be wings level ALL THE TIME.
Curious to hear some definitions of "Vmca", "Crtical Speed" and "Minimum Control Speed"
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I don't think anybody is arguing that it should be wings level ALL THE TIME.
Curious to hear some definitions of "Vmca", "Crtical Speed" and "Minimum Control Speed"
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A and C - Not quite. What used to be referred to as Vmca by the British Civil Airworthiness Requirements (BCARS) has, under JARs become Vmc (Minimum Control Speed). The definition in JAR 23.149 is similarto, if a little shorter than the definition in FAR 23.149 and is as follows:
JAR 23.149 - Minimum control speed
Date: March 11, 1994
(a) VMC 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 thereafter maintain straight flight at the same speed with an angle of bank not more than 5°. The method used to simulate critical engine failure must represent the most critical mode of powerplant failure with respect to controllability expected in service.
(b) VMC for take-off must not exceed 1.2 VS1, (where VS1 is determined at the maximum take-off weight) and must be determined with the most unfavourable weight and centre of gravity position and with the aeroplane airborne and the ground effect negligible, for the take-off configuration(s) with -
(1) Maximum available take-off power initially on each engine;
(2) The aeroplane trimmed for take-off;
(3) Flaps in the take-off position(s);
(4) Landing gear retracted; and
(5) All propeller controls in the recommended take-off position throughout.
(c) For all aeroplanes except reciprocating engine-powered aeroplanes of 2730 kg (6000 lb) or less maximum weight, the requirements of sub-paragraph (a) must also be met for the landing configuration with -
(1) Maximum available take-off power initially on each engine;
(2) The aeroplane trimmed for and approach with all engines operating at VREF at an approach gradient equal to the steepest used in the landing distance demonstration of JAR 23.75;
(3) Flaps in the landing position;
(4) Landing gear extended; and
(5) All propeller controls throughout in the position recommended for approach with all engines operating.
(d) At VMC, the rudder pedal force required to maintain control must not exceed 667.5 N (150 lb) and it must not be necessary to reduce power of the operative engine. During the manoeuvre the aeroplane must not assume any dangerous attitude and it must be possible to prevent a heading change of more than 20°.
You will note that there are no variables involved - flap setting, power, propeller controls, etc. are all fixed resulting in a single speed being declared for each aeroplane.
Critical speed, on the other hand, is defined as "The lowest possible speed on a multi-engined aircraft at which, at a constant power setting and aircraft configuration, the pilot is able to maintain a constant heading after failure of one or more engines on one side" The Critical Speed (but not the Minimum Control Speed) can be affected by many things including, for example, the thrust from the live engine, the position of the centre of gravity, asymmetric drag, the aircraft configuration (flaps, slats, gear, etc.), slipstream, asymmetric blade effect, turbulence, altitude, temperature, rudder trim position and the strength and skill of the pilot.
The Minimum Control Speed for the PA34, for example, is quoted in the POH as 68 knots. The Critical Speed, however, can be anywhere between about 70 knots in the worst configuration (wings level, windmilling propeller, full rudder trim applied, aft centre of gravity, etc) and something less than the stalling speed in the best configuration.
Thus there is a significant difference between Vmc/Vmca (fixed) and Critical Speed (variable).
JAR 23.149 - Minimum control speed
Date: March 11, 1994
(a) VMC 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 thereafter maintain straight flight at the same speed with an angle of bank not more than 5°. The method used to simulate critical engine failure must represent the most critical mode of powerplant failure with respect to controllability expected in service.
(b) VMC for take-off must not exceed 1.2 VS1, (where VS1 is determined at the maximum take-off weight) and must be determined with the most unfavourable weight and centre of gravity position and with the aeroplane airborne and the ground effect negligible, for the take-off configuration(s) with -
(1) Maximum available take-off power initially on each engine;
(2) The aeroplane trimmed for take-off;
(3) Flaps in the take-off position(s);
(4) Landing gear retracted; and
(5) All propeller controls in the recommended take-off position throughout.
(c) For all aeroplanes except reciprocating engine-powered aeroplanes of 2730 kg (6000 lb) or less maximum weight, the requirements of sub-paragraph (a) must also be met for the landing configuration with -
(1) Maximum available take-off power initially on each engine;
(2) The aeroplane trimmed for and approach with all engines operating at VREF at an approach gradient equal to the steepest used in the landing distance demonstration of JAR 23.75;
(3) Flaps in the landing position;
(4) Landing gear extended; and
(5) All propeller controls throughout in the position recommended for approach with all engines operating.
(d) At VMC, the rudder pedal force required to maintain control must not exceed 667.5 N (150 lb) and it must not be necessary to reduce power of the operative engine. During the manoeuvre the aeroplane must not assume any dangerous attitude and it must be possible to prevent a heading change of more than 20°.
You will note that there are no variables involved - flap setting, power, propeller controls, etc. are all fixed resulting in a single speed being declared for each aeroplane.
Critical speed, on the other hand, is defined as "The lowest possible speed on a multi-engined aircraft at which, at a constant power setting and aircraft configuration, the pilot is able to maintain a constant heading after failure of one or more engines on one side" The Critical Speed (but not the Minimum Control Speed) can be affected by many things including, for example, the thrust from the live engine, the position of the centre of gravity, asymmetric drag, the aircraft configuration (flaps, slats, gear, etc.), slipstream, asymmetric blade effect, turbulence, altitude, temperature, rudder trim position and the strength and skill of the pilot.
The Minimum Control Speed for the PA34, for example, is quoted in the POH as 68 knots. The Critical Speed, however, can be anywhere between about 70 knots in the worst configuration (wings level, windmilling propeller, full rudder trim applied, aft centre of gravity, etc) and something less than the stalling speed in the best configuration.
Thus there is a significant difference between Vmc/Vmca (fixed) and Critical Speed (variable).
Guest
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Watford,
I am not really confusing Critical Speed with Vmca as Critical Speed is not in our vocabulary in Australia. Our teaching techniques seem to follow a different tack to the UK. Vmc as defined by FAR 23.149 forms the basis of ‘control' discussion and understanding. But when expanding on that principle the various factors and considerations impacting on Vmc are dealt with. Particularly since during the Air Exercise, the minimum speed at which control is maintained will not necessarily coincide exactly with the red radial. Your methods on the other hand seem to be rather pedantic in teaching Vmc coincides exactly with the red radial and any variation is another speed termed ‘Critical Speed' which may be any speed. I probably prefer the Australian approach.
But my main concern in any initial or subsequent endorsement session revolves around the wings level -v- five degree bank towards the live engine. A point raised in the initial post above by Bear Cub (G and N registrations). In any situation where altitude is a priority then the bank technique is imperative to achieve both performance and a margin of control. In cruise it may be optional, and irrelevant in descent. An engine failure during a descending turn may require momentarily levelling the wings and centring the ball to identify the failed engine. However the student must be able to demonstrate satisfactory control with bank to meet the handling criteria of the endorsement.
Regards,
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dragchute
email: [email protected]
I am not really confusing Critical Speed with Vmca as Critical Speed is not in our vocabulary in Australia. Our teaching techniques seem to follow a different tack to the UK. Vmc as defined by FAR 23.149 forms the basis of ‘control' discussion and understanding. But when expanding on that principle the various factors and considerations impacting on Vmc are dealt with. Particularly since during the Air Exercise, the minimum speed at which control is maintained will not necessarily coincide exactly with the red radial. Your methods on the other hand seem to be rather pedantic in teaching Vmc coincides exactly with the red radial and any variation is another speed termed ‘Critical Speed' which may be any speed. I probably prefer the Australian approach.
But my main concern in any initial or subsequent endorsement session revolves around the wings level -v- five degree bank towards the live engine. A point raised in the initial post above by Bear Cub (G and N registrations). In any situation where altitude is a priority then the bank technique is imperative to achieve both performance and a margin of control. In cruise it may be optional, and irrelevant in descent. An engine failure during a descending turn may require momentarily levelling the wings and centring the ball to identify the failed engine. However the student must be able to demonstrate satisfactory control with bank to meet the handling criteria of the endorsement.
Regards,
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dragchute
email: [email protected]
