General Principles - Landing — Page 289, Lesson 363

I want to walk you through the microburst — the single most hazardous weather phenomenon to an aeroplane close to the ground, particularly during the landing phase. Let's start with the warning signs, because a microburst gives you clues before it hits you. The most obvious clue is what we call virga, also known as fallstreaks. These are the tendrils of rain you see beneath a storm cloud. The key definition here: virga is precipitation that is falling but evaporating before it reaches the ground. So you see rain streaks hanging under the cloud, but nothing actually reaches the surface. That's your advance warning that a microburst may be developing. Now, let's walk through what happens when you actually encounter a microburst on approach, using the flight path in Figure 6.10. Initially, you'll see an increase in airspeed and a rise above the approach path. Why? Because you're flying into an increasing headwind — the outflow from the microburst is blowing toward you. On modern aeroplanes, a windshear alert is usually given at this point. Treat that alert as the precursor to the microburst. Here's the critical decision point: the moment you get that windshear alert, abandon any hope of a stabilized approach and initiate a missed approach immediately. Without hesitation. Increase power to go-around power, raise the nose, and fly the missed approach procedure. Typically the nose attitude will be about 15 degrees, and you hold the control column against the buffet or the stick shaker — that's the stall warning. You're riding right on the edge of the stall to get maximum performance. Now here's the trap. That initial bonus of increased airspeed gets rapidly eroded as you fly into the downdraught. Airspeed falls, and the aeroplane may start to descend despite high power and high pitch angle. The aeroplane is being pushed down by the downdraught faster than your engines can climb you out. Then comes the most critical point: the moment you start to encounter the tailwind on the other side of the microburst. Your rate of descent may actually reduce, but the airspeed continues to fall, and any height loss now breaks into your obstacle clearance margins. At this stage you need maximum thrust, and you keep the nose attitude high, right on the stall warning margins, to escape the effects of the tailwind. Let me be blunt about the stakes here. Had the go-around not been initiated at that early warning stage — at the windshear alert — it is highly unlikely the aeroplane would survive the encounter. That's how serious this is. Now, the broader lesson. Windshear, heavy rain, poor visibility, runway contamination, and microbursts are all hazards very closely associated with thunderstorms. So the advice is simple and absolute: never land with a thunderstorm at or in the immediate vicinity of the aerodrome. Delay the landing, or consider diverting. There are too many cases to list of pilots who attempted to land in bad weather and sadly perished along with many passengers. Most of these occurrences were avoidable had due account been taken of the situation. So the takeaway chain is: recognise the virga as the clue, treat the windshear alert as the trigger to go around immediately, ride the stall warning with maximum thrust through the downdraught and tailwind, and above all — don't put yourself in that position in the first place when a thunderstorm is near the field.