A pilot on final approach receives a PIREP from a preceding aircraft: 'Airspeed increased 20 knots then decreased 25 knots within a 500-foot altitude change on short final.' Based on FAA criteria, this report should be classified as which hazard, and what action should the pilot take?
- ACarburetor icing; the pilot should apply carburetor heat and continue the approach
- BLow-level wind shear; the pilot should consider a go-around or delay the approach and notify ATC
- CClear air turbulence; the pilot should climb to a higher cruise altitude to avoid it
- DLight chop; continue the approach as normal since airspeed fluctuations under 30 knots are insignificant
Show answer & explanationAnswer & explanation
Correct answer: B. Low-level wind shear; the pilot should consider a go-around or delay the approach and notify ATC
A rapid airspeed gain followed by a significant loss (here 25 knots) within a short altitude change near the ground is a classic signature of low-level wind shear, which the FAA generally considers significant when airspeed fluctuations reach or exceed about 15 knots. This hazard is especially dangerous close to the ground on approach, and pilots should consider executing a go-around and reporting the encounter to ATC.
Why the other options are wrong
- A. Carburetor icing affects engine power gradually and would not produce this rapid airspeed signature tied to altitude change.
- C. Clear air turbulence occurs typically at higher altitudes in clear skies, not on short final near the surface.
- D. A 25-knot airspeed loss is well above the threshold considered insignificant and should not be dismissed.
Low-Level Wind Shear (LLWS)
A rapid change in wind speed and/or direction over a short distance near the surface, often producing rapid airspeed gains followed by losses (or vice versa) that are hazardous during approach and departure.
- Often associated with thunderstorms, fronts, or temperature inversions
- Airspeed fluctuations of 15 knots or more are considered significant
- Go-around is the recommended response if shear is suspected on approach
Memory trick: Gain then lose airspeed near the ground — shear's the word!