Total Air Temperature — Page 140, Lesson 166

Right, we’ve just opened the performance section of your ATPL ground training, and the very first thing we need to do is build a shared vocabulary. This whole chapter is a list of the standard speed and weight definitions that every performance calculation hangs on. I want to walk you through them in groups, because they’re not random letters — each one is a precise, certified limit or reference point that you’ll use in the cockpit and in the exam. Let’s start with the take-off and runway group, because these are the ones you’ll meet first on every departure. TOD is Take-off Distance — and note that the same three letters also stand for Top of Descent, so context tells you which one we mean. TODA is Take-off Distance Available, which is the physical runway length you actually have, including any clearway. TODR is Take-off Distance Required — that’s what the aircraft needs to get airborne and climb to the screen height. TOGA is Take-off/Go-around thrust, the maximum thrust setting you select for take-off or for a go-around. TOR is Take-off Run, and TORA is Take-off Run Available — the runway length available for the ground roll. TORR is Take-off Run Required. TOSS is Take-off Safety Speed, and TOW is Take-off Weight. Now the V-speeds — these are the heart of the chapter. V1 is the Decision Speed. That’s the speed at which, if an engine fails, you must continue the take-off; below V1 you stop, above V1 you go. V2 is the Take-off Safety Speed, the climb speed you must reach by the screen height with one engine failed. V2MIN is the Minimum take-off safety speed. V3 is the All engines operating steady initial climb speed. V4 is the All engines operating steady take-off climb speed. Then we have the control and structural speeds. VA is Design Manoeuvring Speed — the speed at which full control deflection won’t overstress the airframe. VEF is the assumed speed of engine failure, the speed used in the performance calculations. VFE is the maximum flap extended speed. VFTO is Final take-off speed. VGO is the lowest decision speed from which a continued take-off is possible within the TODA with one engine inoperative. VLE is the maximum speed with landing gear extended. VLO is the maximum speed at which the landing gear may be lowered. VLOF is Lift-off speed. VMBE is Maximum brake-energy speed — exceed this and the brakes overheat. VMC is Minimum control speed with the critical power unit inoperative. VMCA is Minimum control speed in the air, in the take-off climb. VMCG is Ground minimum control speed, at or near the ground. VMCL is Landing minimum control speed, on the approach to land. VMO is the maximum operating speed. VMU is the minimum unstick speed. VNE is Never exceed speed. VP is Hydroplaning or Aquaplaning speed. VR is Rotation Speed. VRA is the turbulence speed or rough air speed. VREF is the reference landing speed, which replaced the old VAT speed. VS is the stalling speed or minimum steady flight speed at which the aeroplane is controllable. VSR is the reference stalling speed, assumed to be the same as VS1g. VSR0 is the reference stalling speed in the landing configuration. VSR1 is the reference stalling speed in the specified configuration. VS1g is the one-g stall speed, at which the aeroplane can develop a lift force normal to the flight path equal to its weight — again, assumed the same as VSR. VS0 is the stalling speed with flaps at the landing setting. VS1 is the stalling speed for the configuration under consideration. VSTOP is the highest decision speed that an aeroplane can stop within ASDA. VX is the speed for the best gradient or angle of climb. VY is the speed for the best rate of climb. And VZF is the minimum safe manoeuvring speed with zero flap. Finally, the weight and symbol group. WAT is Weight-altitude-temperature, the limiting combination that restricts take-off weight. ZFW and ZFM are Zero fuel weight and zero fuel mass. Then the Greek symbols: γ is the climb or descent angle, µ is the runway friction coefficient, θ is the aircraft attitude, and ρ is air density. Now, I want you to notice the pattern here — every one of these is a certified reference, not a suggestion. When you calculate take-off performance, you’re checking that your required distances fit inside the available ones, and that your decision speed V1 sits correctly between VMCG and VMBE. That’s the relationship you’ll see in Figure 14.2 — V1 cannot be allowed to be less than VMCG, because an engine failure below that speed means you can’t maintain directional control on the ground. So as we move through the chapter, every time I mention a V-speed, you’ll know exactly which limit it represents. That’s the foundation we’re building right now.