Aero: Learning Objectives – Wake Turbulence and Wind Shear – Flashcards

- Spanwise airflow that moves around the wingtip creates both induced drag and wingtip vortices. - Wingtip Vortices - spiraling masses of air that are formed at the wingtip when an airplane produces lift - Wake Turbulence is a disturbance due to another aircraft's wingtip vortices. - Vortices may instantly change the direction of the relative wind and cause one or both wings of the trailing airplane to stall, or disrupt airflow in the engine inlet inducing a compressor stall. - The diameter of the core of the vortex is about 1/4 of the generating aircraft's wingspan. As vortices trail behind the aircraft, they remain within about 3/4 of the generating aircraft's wingspan. They sink at a rate of 400-500 ft per minute and level off about 900 ft below the flight path of the generating airplane. - Vortices will lose strength and break up after a few minutes. - The wake turbulence zone is the region behind the aircraft containing the trailing vortices, and the region between them.

- Weight - To maintain level flight, a heavier airplane must produce more lift, and will therefore have a greater pressure differential at the wingtip where the vortex is created. Weight is the most significant factor in the strength of wingtip vortices. - Airspeed - Since induced drag is dominant at lower airspeeds, a slower aircraft will have stronger vortices. A faster aircraft will spread the vortices energy over a greater distance, reducing the effect of the vortex. - Configuration - If flaps are lowered, more lift is created at the wing root, which decreases the pressure differential at the wingtip. The greatest vortex strength occurs when the generating airplane is heavy, slow, and clean.

- Loss of control due to induced roll. Wing Vortices generate sufficient airflow to exceed the roll control capability of an airplane flying into the vortex unless the wingspan and ailerons of the encountering airplane extend beyond the rotational flow of the vortex. Pilots of short wingspan airplanes, even of the high performance type, must be especially alert to vortex encounters. The most significant factor affecting your ability to counteract the roll induced by the vortices is the relative wingspan between the two airplanes. - Induced Flow Field is created by the interactions of both vortices resulting in a downwash, between the vortices, of up to 1500 ft per min. This can be disastrous to an aircraft that is already descending at a low power setting. - Downwash from hovering helicopters similar to other prop/jet blasts from aircraft. Because of the extreme downwash, small aircraft should avoid operating within 3 rotor diameters of any hovering helicopter. - When a helicopter is in forward flight, it produces twin vortices similar to wingtip vortices. Therefore, pilots should give helicopters the same spacing consideration as conventional airplanes of similar size and weight.

- Minimum Takeoff Spacing - 2 minutes behind a heavy aircraft (over 255,000 lbs) or large aircraft (41,000-255,000 lbs) - Minimum Landing Spacing - 3 minutes behind a heavy aircraft (over 255,000 lbs)

- Follow minimum spacing requirements - Taking off after a larger plane departs ahead of you - ensure your rotation is complete at least 300 ft prior to the larger airplane's point of rotation and conduct your climb-out to remain above their flight path - Taking off after a larger aircraft has landed - rotate at a point forward of where the larger aircraft's nosewheel touched down - Larger airplane performing a touch-and-go - observe minimum spacing requirement

- Landing behind a larger airplane - besides observing the minimum spacing requirement, stay at or above the larger airplane's final approach path and land beyond its nosewheel touchdown point - Landing behind a larger aircraft that has just departed - ensure your touchdown point is prior to the larger aircraft's rotation point

A sudden change in wind direction and/or speed over a short distance in the atmosphere

Takeoff - Wind shear which increases headwind by 20 knots results in an increase of dynamic pressure by 20 knots - IAS is directly proportional to dynamic pressure, so IAS will increase by 20 knots - Increase in IAS results in an increase in lift and therefore causes an initial increase in performance - 20 knot increase in headwind reduces groundspeed by 20 knots resulting in a steeper angle of climb - As long as a proper climb attitude is maintained, a wind shear with an increasing headwind component on takeoff does not pose a serious threat. Landing - A transition from a tailwind to 0 wind results in an increase in performance - Increases lift and causes the aircraft to pitch up and rise above the glidepath - If the pilot is slow to recognize the wind shear, a steep fast approach can quickly develop which can result in significantly longer landing distances - An overly aggressive correction can lead to a high rate of descent with a dangerously low power setting - A higher power setting is required after the shear than before the shear to maintain glidepath due to the slower groundspeed. You must eventually add more power than was removed to stabilize on the glidepath.

Takeoff - Windshear which decreases the IAS by 25 knots causes a significant decrease in performance - The drop in dynamic pressure reduces lift and also results in a shallower angle of climb - Rapid drop in airspeed can plane an aircraft near stall speed - An increase in AOA in this situation will probably result in an approach to stall indication and possibly a stall Landing - Windshear that decreases its IAS - Causes aircraft to pitch down and descend below the glide path - Pilot counters this by adding power and raising the nose, but may over correct and rise above the glide path - Once back on glide path, a lower power setting will be required to compensate for the higher ground speed and new rate of descent within the new body of air - Higher rate of descent dictated by a higher groundspeed so the pilot must eventually reduce power by more than the amount added to stabilize on the glide path

- Increasing Performance Wind Shear during Takeoff - Ground speed will be reduced resulting in a steeper angle of climb. As long as a proper climb attitude is maintained a wind shear with an increasing headwind component on takeoff does not pose a serious threat. - Decreasing Performance Wind Shear during Takeoff - A rapid drop of airspeed can place an aircraft near stall speed. An increase in angle of attack in this situation will probably result in an approach to stall indication and possibly a stall. - Increasing Performance Wind Shear during Landing - A higher power setting is required after the shear than before the shear to maintain glidepath. You must eventually add more power than was removed to stabilize on the glidepath. If too much power is removed, combined with an overly aggressive correction back to the original glidepath, there may be insufficient altitude for the pilot to recover, resulting in landing short of the runway. If a strong wind shear is encountered at low altitudes a wave-off/go-around can be executed if necessary. - Decreasing Performance Wind Shear during Landing - A higher rate of descent results from a higher ground speed. You must eventually reduce power by more than the amount added to stabilize on the glide path. If a strong decreasing performance wind shear is encountered at a very low altitude, or if a pilot is slow to recognize the situation, there may be sufficient time and power to overcome the resulting loss of lift and landing short of the runway.

For Takeoff: - Use the longest suitable runway. Consider crosswind, obstacles, runway surface conditions, etc., when selecting the runway. - Use takeoff flaps, but delay rotation (V)ROT by the amount of predicted wind shear (up to 10 additional knots). Notice, this addition applies to increasing performance wind shear. - Rotate to normal climb attitude at increased (V)ROT, abort if possible. For Landing: - Set flaps to takeoff and increase approach speed by the amount of wind shear potential (up to 10 knots above normal). Again, notice this addition applies to increasing performance wind shear. By setting the flaps to takeoff, there will be less drag and the aircraft will be able to accelerate more quickly. - Establish the proper approach pitch, trim, and power settings by 1000 AGL. Resist the temptation to make large power reduction. Keep in mind that increased landing speed means longer landing distances.

- If a moderate to strong wind shear is expected, delay your takeoff or landing until the shear condition no longer exists. - Anytime wind shear is experienced, pilots should consider going around. If airborne and unable to delay, consider diverting to a place with more favorable conditions. - Strong shears like those associated with microburst activity must be avoided at all cost. An aircraft encountering a microburst will initially experience a strong, increasing performance wind shear. This will cause the aircraft to pitch up and climb causing the pilot to apply nose down stick. Soon after, the aircraft will experience a strong, decreasing performance wind shear. Combined with the nose-down stick applied by the pilot, the decreasing performance wind shear can result in impacting the terrain.