Windshear, particularly low altitude windshear encounters, are of significant importance because it can place the flight crew in a situation where the maximum performance capability of the aircraft is required. Windshear encounters below 500ft are the most threatening because there is very little time or altitude to respond and recover from an inadvertent encounter.

Knowledge of how windshear affects performance can be essential to a successful recovery manoeuvre following an inadvertent windshear encounter.

The wind component is mostly horizontal below 500ft. Horizontal windshear may improve or degrade vertical flight path performance. Windshear that improves performance will be first indicated in the cockpit by an increased airspeed. This type of windshear may be a precursor of a shear in the reverse sense that will decrease airspeed and degrade vertical flight path performance.

The airspeed will decrease if the tailwind increases faster than the aircraft is accelerating. As the airspeed decreases, the aircraft normally tends to pitch down to maintain or regain the in-trim speed. In some cases during take-off with an aft C of G the aircraft pitch response to a decreasing airspeed may be small or even nose up. The magnitude of the pitch change is a function of the following

  - Aircraft configuration
  - Weight
  - Speed
  - C of G
  - Thrust
  - Severity of airspeed change

If an attempt is made to regain lost speed by lowering the nose, the combination of decreasing airspeed and decreasing pitch attitude produces a high rate of descent. In some circumstances as little as 5 seconds may be available to recognise and react to a degrading vertical flight path.

In critical low altitude situations, flight path control must be maintained through the use of pitch attitude and thrust. An increase in pitch attitude will increase lift and improve flight path angle. In other words trading speed for altitude. This trade-off combined with maximum thrust will utilise the full aircraft performance.

The flight crew must be aware of the normal values of airspeed, altitude, rate of climb, pitch attitude and control forces. Unusual control forces may be required to maintain or increase pitch attitude when airspeed is below the in-trim speed. If significant changes in airspeed occur and unusual control forces are required, this should alert the crew to a possible windshear encounter and they must be prepared to react promptly.

Windshears which exceed the performance capabilities of commercial transport aircraft such as the Boeing 757 have been observed below 500ft. The flight crew should search for any clues to the presence of windshear along the intended flight path.

Crews should carefully review all available information such as pilot reports of windshear or turbulence, low level windshear reports and weather reports - especially thunderstorms and 'virga'.

Windshear that produces uncommanded airspeed changes of 15 knots or more is regarded as severe. Areas of known windshear should be avoided and pilots should delay take-off or discontinue an approach until conditions have improved wherever necessary.

Windshear encounters should be reported precisely and promptly to assist other pilots. Accurate pilot reports of windshear can be a valuable clue as to the presence and severity of windshear conditions.

If windshear is suspected, flight crews should be especially alert to any of the danger signals and be prepared to take prompt recovery actions.

Maximum thrust should be used; the use of reduced take-off thrust is not permitted in windshear conditions. This restriction does not necessarily apply to windshear associated solely with strong surface winds in which cases the Captain has the discretion to use reduced thrust. If practical the longest suitable runway available should be used, provided it is clear of areas of known windshear.

The flight director should not be relied upon during take-off in suspected windshear conditions. The attitude director is the primary reference for pitch attitude.

Crews should be alert for airspeed fluctuations during take-off and initial climb. Such fluctuations may be the first indication of windshear. Control forces may be different from those expected, especially if airspeed is below the in-trim speed.

Crews should know the all-engine initial climb pitch attitude. Rotation should occur at the normal rate to this attitude for all take-offs when engine failure is not a factor. Reductions from the initial climb attitude should be avoided until terrain and obstruction clearance is assured. Smooth steady control of pitch attitude is essential.

Crew co-ordination and awareness are very important. Close monitoring of the flight instruments is imperative, and the non-handling pilot should be especially aware of these and call out any deviations from normal values.

The stick shaker must be respected at all times. If it is activated, pitch attitude should be reduced just enough to stop the stick shaker. Flight with intermittent stick shaker operation may be required to maintain a positive rate of climb during a windshear encounter.

Flight crews should use all available means that might alert them to the presence of windshear, including visual clues, pilot reports and flight instruments.

An increment to Vref should be made of as much airspeed as the aircraft is expected to lose - up to a maximum of Vref+15 knots. If the expected speed loss exceeds 15 knots the approach should not be attempted.

A comparison of the reported surface wind with the IRS wind readout will give an indication of the degree of windshear to be expected in the later stages of the approach.

Large thrust reductions or trim changes in response to a sudden airspeed increase should be avoided as these may be followed by a decrease in airspeed.

In windshear conditions flight director commands should be checked against ADI, VSI and altimeters. These instruments are the primary references for vertical flight path control.

Crew co-ordination and awareness are very important, particularly at night or in marginal weather conditions, to ensure immediate recognition of a deteriorating flight path. The VSI, altimeters and glideslope displacement (if available) should be closely monitored. The non-handling pilot should be especially aware of these and call out any deviations from normal values.
Use of autopilot and autothrottle during approach may provide more monitoring capability and recognition time for the flight crew.

It cannot be overstressed that the avoidance and precautionary action mentioned above are paramount in ensuring that a windshear encounter is unlikely, however the following actions should be taken when preventative action is not successful, or whenever flight path control becomes marginal below 500ft. This applies during take-off or landing.

The flight crew should use all available cockpit information when determining whether the flight path control is marginal. This determination is subjective and based on the pilot's judgement of the situation. As a guide, marginal flight path control may be indicated by uncontrolled changes from the normal steady state flight conditions in excess of:

  - 15 knots indicated airspeed
  - 500 fpm vertical speed
  - 5 degrees pitch attitude
  - 1 dot glideslope displacement

Whenever flight path control becomes marginal below 500ft AGL, or when the "WINDSHEAR" or "PULL UP" warning occurs, simultaneously:-

- Call "Windshear Go-around"
- Complete the rest of "Windshear" recall items (QRH)

The call of "Windshear Go-around will advise the non-handling pilot that the usual go-around procedure does not apply and that speeds and pitch attitudes may not be normal for a go-around.

Pitch attitudes of up to 25 degrees may be required to minimise height loss, this will involve sacrificing speed to ensure a positive rate of climb. In all cases the pitch attitude that results in intermittent stick shaker is the upper pitch attitude limit. Large thrust increases may result in a nose up pitching tendency requiring forward column pressure and trim. Where fitted the Pitch Limit Indicator (amber "eyebrows" on the ADI) display the pitch attitude at which stick shaker activation will occur, this is only displayed when the flaps are extended.

Vertical speed and height should be monitored at all times. The flap or gear position should not be changed, nor should any attempt be made to regain lost airspeed, until ground contact is no longer a factor.

The turbulent air penetration speed (290 knots / 0.78 Mach) was selected to provide low / high speed buffet margins in severely turbulent air.

When encountering severe turbulence, maintain wings level and smoothly control attitude. Use the ADI as the primary instrument. In extreme vertical wind currents, large attitude changes may occur. Do not use sudden or large control inputs.

After establishing the trim setting for penetration speed, do not change stabiliser trim. Allow altitude and airspeed to vary and maintain attitude. Set thrust for penetration speed and avoid large thrust changes. Ignition should be set to 'CONT'.

The autopilot may remain engaged as long as performance is satisfactory. Otherwise disengage the autopilot and fly the aircraft manually. Autothrottle should be OFF in severe turbulence.

Normally no changes to cruise altitude or airspeed are required when encountering moderate turbulence. If operating at cruise thrust limits it may be difficult to maintain cruise speed.

The autothrottle on the Boeing 757 is more sophisticated than some pilots may be used to on other aircraft types. It should be noted that on approach the autothrottle will remove power at only half the rate that it will apply it when it recognises a speed error. The result of this is a built in ratchet effect when used in gusty conditions. As the autothrottle recognises a speed loss it will apply extra power to drive the speed up to the cursor again (indeed it is likely to drive it past the cursor and the speed will settle slightly above the selected target value). If conditions are then stable for a period the autothrottle will reduce power until the desired speed is achieved. On a bumpy day this does not happen and a reasonably stable situation is achieved with the airspeed somewhat above the selected setting. This applies with the autopilot engaged or in manual flight.

Experience has shown that in testing conditions the speed control achieved by autothrottle is often better than can be achieved by manual thrust control.

The GPWS "PULL UP" warning occurs when unsafe closure with terrain occurs.

Anytime that a GPWS "PULL UP" is activated and the aircraft is below MSA or when safe terrain clearance is in doubt, disengage the autopilot and autothrottle, aggressively position the throttles forward to ensure maximum thrust is attained and rotate smoothly at a normal rate towards an initial pitch attitude of 15 degrees. These are the same initial actions as for the recovery from an inadvertent windshear encounter. Be prepared to trim to overcome the push force after adding thrust. Pitch attitudes in excess of 15 degrees may be required to avoid terrain. If the GPWS warning continues, continue to rotate while monitoring airspeed and vertical speed.

Sustained climbs at speeds lower than flaps up manoeuvre speed (Vref+80) do not result in an appreciable climb gradient improvement; however if required to avoid terrain, continue to rotate until stock shaker or buffet occurs, then reduce the pitch attitude just enough to silence the shaker or fly out of the buffet. Smooth, steady control of pitch attitudes will ensure that high pitch rates do not develop and will avoid an overshoot of the pitch attitude at which stall warning is initiated.

Respecting stick shaker or buffet will ensure that appropriate manoeuvre and stall margins are maintained. As mentioned previously, the Pitch Limit Indicator (where fitted) shows stick shaker activation attitude whenever flaps are extended.

Do not attempt to regain lost airspeed until ground contact is no longer a factor.