General Principles - Cruise — Page 241, Lesson 289

We're moving into the cruise phase of flight now, and the first thing I want to nail down is a speed we haven't fully defined yet: the true ground speed, abbreviated TGS, or just GS. Think of it this way. True airspeed is how fast the aeroplane is moving through the air mass around it. But the ground doesn't care about the air mass — the ground is fixed. So the true ground speed represents the aeroplane's speed within a fixed ground reference system. Put simply, it is the aeroplane's velocity over the ground. Here's the key relationship: the true ground speed is equal to the true airspeed plus or minus the wind component. Let me give you the two cases. If there's a tailwind, the aeroplane's speed over the ground will increase by a value equal to the speed of that tailwind. So if the true airspeed is 250 knots and the tailwind is 20 knots, the true ground speed is 270 knots. Conversely, if there's a headwind, the speed over the ground is reduced by a value equal to the speed of the headwind. So if the true airspeed is 250 knots and the headwind is 30 knots, the true ground speed is 220 knots. Now, what if you don't already know the headwind or tailwind component? You can work those out using a flight navigation computer, or by using graphs or tables in the aeroplane flight manual. And for our purposes, CAP 698, Section 4, Page 4, Figure 4.1 can also be used. Now I want to shift gears, because there's a physical effect that becomes very important as we push to higher speeds in cruise. At speeds higher than about 220 knots, some of the energy of the aeroplane goes into compressing the air ahead of it, and locally increasing the density of that air. That's compressibility. Compressibility affects the amount of drag force on the aeroplane, and the effect becomes more important as speed increases. Let me explain what's happening physically. As the aeroplane moves through the air, it makes noise simply by disturbing the air. That noise emanates outwards in the form of pressure waves. These pressure waves stream out away from the aircraft at the speed of sound, in all directions — acting just like the ripples through water when a stone is dropped in the middle of a still pond. If the aeroplane were theoretically stationary, you'd see those waves radiating evenly in all directions around it. But here's the crucial part. As the aeroplane approaches the speed of sound, it actually starts catching up with its own pressure waves in front of it. Those pressure waves pile up and turn into one big pressure shock wave. That shock wave causes a loud bang — that's the sonic boom. And importantly for performance, that shock wave actually buffets the aeroplane and decreases the lift force. So keep these two threads together: ground speed is simply true airspeed adjusted for the wind component, and then as we climb toward the speed of sound, compressibility starts changing the drag picture, and eventually the pressure waves ahead of the aircraft coalesce into a shock wave that costs us lift.