Class A Aircraft - Take-off — Page 383, Lesson 473

Let's start with the minimum unstick speed, VMU. This is the slowest calibrated airspeed at which the aeroplane can safely lift off the ground and continue the take-off. Calibrated airspeed, remember, is the indicated airspeed corrected for instrument and position error — it's the speed we actually use for performance calculations. So VMU is the absolute lowest speed at which the aeroplane can physically get airborne and keep climbing. Now, here's the surprising part. Even though VMU is the lowest speed at which the aeroplane can lift off, in actual operating conditions we never lift off at VMU. We always fly the aeroplane so it lifts off at a slightly faster speed. Why? Three reasons. First, VMU is very close to the stall speed, so we're right on the edge of losing lift. Second, at that speed the aeroplane's controllability is very "sloppy" — the controls feel mushy and unresponsive. Third, to actually lift off at VMU requires some fairly dramatic actions, which would be uncomfortable for the passengers. And here's the real kicker — the aeroplane is actually able to lift off at a speed where lift is less than weight. That seems impossible, doesn't it? But here's the physics. So long as the nose can be raised to a high enough attitude, there's a vertical component of thrust. That vertical thrust, together with lift, balances weight. The amount of this vertical thrust is controlled in part by the amount of thrust generated, but also by the amount of nose-up attitude the aeroplane can attain. And that nose-up attitude may be limited by two things: the power of the elevator to push the tailplane down, or the tailplane striking the runway — what we call a tail strike. So you can see why it's unwise in operational conditions to lift off at VMU. The actual speed the aeroplane will lift off at in operational flights is called VLOF, and we'll discuss that speed later. Now let's move to VMCA, the air minimum control speed. VMCA is the minimum flight speed at which the aeroplane is controllable, with a maximum of 5° bank, when the critical engine suddenly becomes inoperative with the remaining engine(s) at take-off thrust. So picture this: you're climbing out, the critical engine fails, and you have to maintain control with the remaining engines at full take-off thrust. The asymmetric thrust from the good engine tries to yaw the aeroplane. VMCA is the slowest speed at which you can still maintain control, using no more than 5° of bank to help counteract that yaw. Although VMCA is the minimum control speed in the air, the factors that affect it can, for the purpose of the exam, be assumed to be the same as for VMCG. Finally, let's look at VR, the rotation speed. VR is the speed at which the pilot initiates action to raise the nose gear off the ground, with the intention of becoming airborne. The pilot pulls back on the control column. This deflects the elevators to create a downward aerodynamic force on the tail. That force rotates the aeroplane about its lateral axis — the axis running wingtip to wingtip — and raises the nose wheel off the ground. Now, VR may not be less than four things. First, it may not be less than V1. Second, it may not be less than 1.05 times VMC. Third, it must be a speed such that V2 may be attained before 35 feet. And fourth, it must be a speed such that if the aeroplane is rotated at its maximum practicable rate, the result will be a VLOF of not less than 1.1 times VMU with all engines operating, or 1.05 times VMU with an engine inoperative. And there's a qualification: if the aeroplane is geometry limited or elevator power limited, those margins reduce to 1.08 times VMU with all engines, and 1.04 times VMU with an engine inoperative. Let me unpack that last one. The 1.1 and 1.05 margins assume the aeroplane can rotate freely. But if the aeroplane is geometry limited — meaning the tail might strike the runway — or elevator power limited — meaning the elevator can't generate enough force — then you can't achieve that full rotation rate, so the margins are reduced to 1.08 and 1.04. So to summarise: VMU is the physical minimum for lift-off, but we never use it operationally. VMCA is the minimum control speed in the air with one engine failed. And VR is the speed at which you actually rotate, and it's constrained by V1, by 1.05 times VMC, by the need to reach V2 before 35 feet, and by the VMU margins we just covered.