Pitch Limit Indication (PLI) was implemented on Boeing Aircraft concurrent with Reactive Windshear Warning during the mid 1980s. This implementation displayed PLI at all flaps-down and did not display when the flaps were up (13752). PLI indicated margin to stick shaker and provided awareness information to the flight crew. Flaps-up PLI was not displayed because it added clutter to the display during a flight regime when flaps-up PLI was not expected to be needed. However, flight crews advised that flaps-up PLI pop up would be useful information during flaps-up conditions at high angles of attack to provide guidance for obtaining the maximum flaps-up climb capability of the airplane.
Flaps-up PLI pop up is now being implemented on aircraft fitted with the Advanced Stall Warning Computers (ASWC) Left and Right PIN 285T1 104-221 and are fitted on the B753 (G-JMAA and G-JMAB). The PLI is normally blanked during flaps-up flight, but will pop up into view when speed falls below either 1.14 times stick shaker speed or the lower amber band on speed tape if fitted. If wing anti-ice is used in flight, PLI pop-up will occur earlier and remains in this condition until after landing, even if wing anti-ice is subsequently selected off. There is no change to the present stick shaker trip points, but flaps-up PLI will pop up during most emergency climb situations. Flaps-up PLI pop up is not expected to occur in a typical flight unless doing manoeuvres near the minimum operating speed.
The stick shaker provides a warning that the airplane may stall unless the airplane angle of attack is reduced by increasing the airspeed and/or reducing the load factor. Stick shaker activation is principally based on vane angle and flap position and is set to precede the 1g stall speed by a regulatory prescribed margin of at least 3%. The stall warning margin is 6% (about 10 kt) for flaps up at low weights and altitudes on the 757-300 with the speedbrakes retracted. The stall warning margin is greater for speedbrakes partially or fully extended.
The PLI symbol on the ADI shows the estimated airplane pitch attitude at which stick shaker activation will occur. For flaps UP, it pops-up into view when the current indicated airspeed slows to 1.14 times the estimated 1g stick shaker speed. At that moment it represents a 0.3g margin (40° bank) to stick shaker. For flaps UP at low altitudes this represents a speed margin of about 25 kt to the 1g stick shaker activation. At the moment the PLI pops-up the PLI symbol should be about 3 degrees above the airplane symbol for flaps UP. The PLI software algorithm is approximate and it is based on the margin between the current indicated vane angle and the stick shaker vane angle. This vane angle margin is then converted to an airplane angle of attack margin. The PLI symbol is this angle of attack margin added to the current airplane pitch attitude.
The speedbrakes are used to increase drag and reduce lift. The lift loss means that the airplane angle of attack must be increased to maintain the same target airspeed. As a result, manoeuvring capability to stick shaker at this airspeed is reduced. For example, at Vref 30+80 the bank capability may be nearly 50 degrees when the speedbrakes are retracted, but closer to 40 degrees when they are fully extended. The bank capability is actually somewhat less than 40 degrees when the speedbrakes are fully extended on some Boeing airplanes, including the 757-300. There is no regulatory requirement to provide 40 degree bank capability with the speedbrakes extended, but 30 degrees has been judged by Boeing to be the minimum that is still operationally adequate. Partial speedbrakes reduce airplane lift and bank capability less than full speedbrakes.
If the Wing Thermal Anti-Ice (WTAI) has been used the stick shaker speed will be increased by about 5 kt for the remainder of the flight. As a result, the PLI would pop-up about 5 kt earlier than if WTAI had never been used.
A forward CG reduces manoeuvring capability and makes the PLI pop-up sooner than when the CG is aft.
A slightly earlier PLI pop-up might also be caused by large steady crosswind/side-slip conditions that could increase the trimmed airplane angle of attack and reduce the manoeuvring capability.
Use of partial speedbrakes or targeting Vref 30+85 would prevent the PLI from popping-up as frequently and ensure that a full bank capability of at least 40 degrees to stick shaker is always available.
Boeing Aeroplane Operating Manual Volume 2 - 757-300 Amendment