LOW LEVEL WINDSHEAR IN APPROACH. POSITIVE VERSUS NEGATIVE - اسأل الطيار ask pilot

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Sunday, October 28, 2018

LOW LEVEL WINDSHEAR IN APPROACH. POSITIVE VERSUS NEGATIVE


Low-level windshear is a sudden change in wind speed or direction. It can be in the horizontal plane, the most common one, or in the vertical plane, in which you will deal with a change in height.
It can be associated with frontal masses, thunderstorms, low-level mountain waves (as usually found in islands) or mechanical turbulence.
We can distinguish two different types of low-level windshear in the horizontal axis. Positive wind shear and negative wind shear. Let's explain the difference between them both.

Disclaimer: Actions that we need to take in windshear situations are very well described in the FCOM of all Companies. Usually when we find an alert, you need to go around. But what happens if we don't find the alert?

Positive windshear is a sudden increase in headwind, making the airspeed rise and making the aircraft fly above the glide slope. If you are told to expect positive windshear on final, you should land with a lower setting of flaps, so as to give more kinetic energy to the aircraft, increasing lift to drag ratio, and at the same time, increasing windshear escape capability. It also increases the approach speed, giving you more margin on stall. The only negative issue that might be expected on landing with a lower setting of flaps could be the runway length.

The key when finding windshear is to anticipate and to take quick corrective actions which will limit the divergence.  Autothrust and autothrottle systems might be too slow to recognize such a situation so, manual flying could be advisable. At first, when finding a positive windshear situation, you will try to counteract it by taking the throttles back, but you have to be aware that jet engines may take up to 8 seconds to give TOGA thrust from idle (spool up time). That is a huge amount of time, so you should be altitude-minded. Maybe at 2000 feet on the ILS you could take the throttles back to idle, but below 1000 feet it is advisable to reduce the throttles just a little bit only in order for them to get back to giving thrust pretty soon. If this results in a temporary high-speed destabilization, the Captain, or crew, will need to make a decision about going around or maintaining that destabilization for more time depending on the Company policy and his/their airmanship. Airbus aircrafts have the GS mini protection on their side, increasing the approach speed in case positive windshear is encountered. In other types of aircrafts, an increase in the final approach speed could help fix this problem.

Negative windshear, on the other hand, is a sudden decrease in headwind, it doesn't necessarily need to be tailwind, just an important drop in headwind. reducing the airspeed and making the aircraft fly below the glide slope. Here you must be aware that you are losing kinetic energy. This situation, from my point of view, is worse than a positive windshear. If you are told to encounter negative windshear on final you should select a lower setting of flaps for the approach, for the same reasons that you did it on a positive wind shear. Be minded that tailwind approaches are performed much better with full flaps, but full flaps have a lower margin to go around if needed, and approach speeds have a lower stall margin than a lower setting of flaps. So, in this case, although you will encounter a situation similar to tailwind approach, you will be better off with a lower selection of flaps.

Here, you will need an increase in thrust in order to counteract the decrease in airspeed, but you will also need to be altitude-minded. Anticipation is also the key here, do not let it diverge too much and corrections will be smaller. If you are not able to stabilize the aircraft before reaching 1000 feet, a go-around is very advisable. But, if you encounter negative windshear below 1000 feet, you will have to ask yourself. Will I be able to stabilize it before 200 feet? If the answer is no, or I do not know, you'd better go around. Below 200 feet, going around with negative windshear could be very dangerous as the lack of airspeed and kinetic energy could crash the aircraft against the runway. I would advise to try to recover some energy through the throttles and try to make the best of it at landing.

You will need to take into account that you will be maintaining the glide slope with pitch, and in this situation, pitch changes are working against you as you increase your angle of attack to maintain glide making it harder on your side to exit the pre-stall situation. Remember that changes in thrust when flying an aircraft with underslung engines require a change in pitch, so abrupt changes in thrust in final approach will destabilize the approach.

Local knowledge of the airdrome we are trying to land and knowledge of windshear is basic in anticipating these situations.

Lastly, I want to introduce the concept of going around in a windshear situation, which defers from a normal go around as you do not change your configuration, that is flaps nor gear, during the initial go around actions. That is done in order to avoid compounding the problem if we raised the flaps and lift was needed, or by raising the gear and you found yourself touching the runway in the goaround and incresing the drag by opening the gear doors. This situation stays until out of the wind shear. So, for a windshear go around, the PF will perform the callout "Windshear, TOGA", but the PM will not touch the aircraft configuration. Once the aircraft is out of the windshear, raising flaps and gear can be performed.

Hope this explanation helps!!

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