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

Let’s start with a speed you’ll see defined early in the take-off performance work: VMU, the minimum unstick speed. VMU 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 use for performance calculations. So VMU is the absolute lowest speed at which the aeroplane can physically get airborne and keep flying. But here’s the key point: even though VMU is the lowest speed at which the aeroplane can lift off, in actual operating conditions we do not lift off at VMU. We fly the aeroplane so it actually 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 the lift envelope. 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. Now here’s the part that seems strange at first: the aeroplane is actually able to lift off at a speed where lift is less than weight. How is that possible? Because, so long as the nose can be raised to a high enough attitude, there is a vertical component of thrust which, together with lift, balances weight. So the thrust isn’t just pushing the aeroplane forward — when the nose is pitched up, part of that thrust acts upward and helps hold the aeroplane in the air. 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 the geometry of the aeroplane and the elevator authority both cap how high we can pitch the nose. That’s why, operationally, we don’t lift off at VMU. The actual speed the aeroplane lifts off at in operational flights is called VLOF, and we’ll discuss that speed later. Next, let’s look at VMCA, also written VMC — the air minimum control speed. This 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 the scenario is: we’re climbing out, the critical engine fails, and we need to maintain control in the air. VMCA is the slowest speed at which we can still do that, using up to 5° of bank to help counter the asymmetric thrust. Although VMCA is the minimum control speed in the air, the factors that affect VMCA can, for the purpose of the exam, be assumed to be the same as for VMCG — the ground minimum control speed. So whatever affects VMCG — things like thrust, flap setting, centre of gravity position — we treat as affecting VMCA in the same way. Finally, 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’s action is to pull back on the control column. That action deflects the elevators to create a downward aerodynamic force. That force rotates the aeroplane about its lateral axis and raises the nose wheel off the ground. So rotation is a deliberate pitch-up manoeuvre, not something that just happens. Now, VR is not a free choice — it may not be less than several things. Let me list them carefully. VR may not be less than V1 — the take-off decision speed. It may not be less than 1.05 times VMC — so 5% above the air minimum control speed. It may not be less than a speed such that V2 — the take-off safety speed — may be attained before 35 ft above the runway. And it may not be less than 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 on that last one: if the aeroplane is geometry limited or elevator power limited — meaning the nose-up attitude is capped by tail strike risk or by elevator authority — then those margins change. They become 1.08 times VMU with all engines operating, and 1.04 times VMU with an engine inoperative. So the picture is: VR is the speed at which we rotate, but it’s constrained from below by V1, by 1.05 VMC, by the need to reach V2 before 35 ft, and by the need to achieve a VLOF that clears VMU by a safety margin — a margin that shrinks if the aeroplane is geometry or elevator limited. Let me show you how V1 relates to VMCG and VMBE — that’s the relationship that underpins why VR can’t go below V1. And here’s the picture of the factors that affect VR. So, to pull it together: VMU is the absolute minimum unstick speed, but we don’t use it operationally because of stall proximity, sloppy control, and passenger comfort — we use VLOF instead. VMCA is the air minimum control speed with 5° bank and one engine failed at take-off thrust. And VR is the rotation speed, bounded below by V1, 1.05 VMC, the V2-before-35-ft requirement, and the VMU margins. That’s the core of Class A take-off speeds.