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Unstabilised Approach: Vectoring to Runways with Significant Tailwind Component
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|Category:||Toolkit for ATC - Stabilised Approach|
Failure to recognise the possible implications for a stabilised approach of vectoring towards a landing runway which has a limiting or near-limiting tail wind component.
Effect of Action on Flight Crew and Their Potential Response
The required rate of descent on final is liable to be close to the maximum rate of descent recommended or permitted at typical stabilised approach gates and may lead to a go around even if the surface tailwind component remains within the applicable AFM limits for a landing.
The figure above shows that a 10-knot tail wind, which in most cases is accepted by ATC in the process of runway configuration choice, for a constant approach speed of 145 knots and at 1500 feet requires rate of descent of 850 feet per minute. At 1000 feet rate of descent is reduced by only 10-20 feet.
Based on the stabilised approach criteria, a rate of descent of maximum 1000 feet per minute is required. The above figure is presented for speed of 145 knots. If we imagine though that the plane, due to approach speed corrections or ATC restrictions, is flying at a speed of 160 knots or more, the rate of descent approaches 1000 feet per minute and the “chances” for an ustabilised approach are increasing.
2Moving closer to the ground, if the rate of descent is too high, the flight crew may attempt to avoid a firm touchdown by commanding a high pitch rate. However, if the resulting lift increase is not sufficient to reduce the rate of descent, a firm touchdown may occur. In addition, the high pitch rate may be difficult to control after touchdown, particularly in the case of a bounce. Also a flare that is too high can result in a combination of decreased airspeed and a long float. Since both increase the aircraft’s pitch attitude, the result is reduced tail clearance.
In such case the following effects may occur:
A simple “rule of thumb” is available to calculate rate of descent for a 3 degree glide path:
Ground speed x 5 = Rate of Descent for a 3° glide path
Example: Ground speed is 160 kts. To stay on 3° glide path the descent rate should be 800 feet per minute (no-wind conditions).
ATC Options to Avoid the Action
- Actively monitor wind velocity forecast and actual trends so as to minimise the exposure of inbound aircraft to significant tail wind components on final approach and for landing.
- Appreciate the difference between airspeed as the sole reference for aircraft speed control and the ground speed seen directly on ATC radar.
- Note that wind velocity on final approach can be quite different to surface measurements in both speed and direction.
- Ensure that in the presence of significant airborne tailwind components, accurate track miles are determined, are reasonable and are communicated to flight crews.
ATC Options to Manage the Consequences
- Do not designate runways with significant tailwind components (e.g. more than 10 knots) as landing runways and in the case of strong surface winds, make it a priority to minimise the tailwind component on landing runways.
- Be prepared offer a change of approach and landing runway if an unexpected increase in tailwind has occurred whilst an aircraft has been descending and ensure that radar vectors to re-position are acceptable to the crew concerned.
- Stabilised Approach
- Non-stabilized Approach After ATC-Requested Runway Change (OGHFA SE)
- Runway Overrun After Unstabilised Approach (OGHFA SE)
- DGAC (France) Publications on Non-Stabilised Approaches
- CFIT Precursors and Defences
- Continuous Descent Final Approach (CDFA)
- Runway Excursions - An ATC Perspective on Unstable Approaches
- Avoiding Unstable Approaches - Important Tips for ATCOs
- Unstable Approaches – ATC Considerations, January 2011