Basic Flight Aerodynamics (PART-31)

                                           

                                      Stall protection: 

An aircraft Stall Warning System is that system which provides the pilot with advance warning of an impending stall. As is the case with most safety related equipment, the requirement for a Stall Warning System will be found within the aviation regulations for the state of Manufacture and the state of the operator for any given aircraft. The required operational parameters of the Stall Warning System on a particular aircraft will, in part, be dependent upon the aircraft weight, capacity and purpose and the regulations under which the aircraft Type of Certificate was issued.

As per the regulations, the stall warning system requirements can be satisfied by the inherent stall characteristics of the aircraft itself or by other appropriate means. Some of the most common stall warning systems are as follows:

Pre-Stall Buffet: In this case, the warning of the impending stall is provided solely by aerodynamic buffet. As the aircraft approaches the stall, the airflow across the upper cambered surface of the wing ceases to flow smoothly, it losses contact with the wing surface and it becomes turbulent. If the turbulent air then flows across the horizontal stabilizer, buffet results. In many aircraft, even some as large as the locked AC-130 spectre, this buffet provides the sole warning of the impending stall.

 Audible Warning:  Stall warning is provided by an electronic or mechanical device that sounds an audible warning as the stall speed is approached. The simplest such device is an air-frame mounted stall warning horn which sounds when the airflow through it occurs at a specific angle. Slightly more sophisticated audible warning devices consist of either a pressure sensor or a moveable metal tab that actuates a switch as the stall is approached. The switch, in turn, activates an audible warning horn. In some installations, the audible warning is provided by a synthetic voice which helps to reduce warning ambiguity.

 Stick Shaker: A stick shaker is a mechanical device that shakes the control column to warn of the onset of stall. A stick pusher may be installed in association with a stick shaker system in aircraft which are susceptible to the deep stall phenomenon. A deep stall affects certain aircraft designs, most notably those with a T-tail configuration, and results in a substantial reduction or loss of elevator authority making normal stall recovery actions ineffective; in many cases, a deep stall might be unrecoverable. The stick pusher is designed to prevent the pilot from allowing the aircraft to enter a stall.

 

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Basic Flight Aerodynamics (PART-30)

                                      Gust lock systems:  

A Flight Control Lock (sometimes referred to as a Gust Lock) is a means by which control surfaces on an aircraft not in flight can be prevented from random movement caused by wind,jet blast or propeller wash and thus possible damage to the surface, attachment points or control system. Most aircraft have some means of preventing or at least limiting flight control surface movement when parked but some light aircraft manufacturers recommend rather informal methods of achieving locking of the aileron and elevator controls by, in one case,Where control locks are provided, they may be fitted externally, in which case they align the control surface with the adjacent structure, be internally fitted to the flight controls or be select-able by means of a lever in the flight deck. Many light aircraft use an elevator/aileron control lock which is fitted to the one of the control columns and there is sometimes a means of preventing rudder pedal movement in a similar way. Some business and smaller transport aircraft which do not have fully powered flight control systems have a gust lock lever in the flight deck which is routinely set whenever the aircraft is parked. Aircraft may additionally be provided with a set of externally-fitted gust locks for use when extreme winds are forecast or during an extended period parked. Control locks are generally not necessary on larger modern aircraft since these normally have powered (usually hydraulically actuated) flying control surfaces and the damping action of the residual hydraulic actuator pressure is sufficient whilst taxiing and for normal in-service parking periods.If Control Locks are engaged specifically as a defence against system damage due to strong winds, then it may be necessary to orient the aircraft nose into wind for maximum effectiveness and to tie down a light aircraft or fully chock all landing gear in the case of larger aircraft.The risk of flight control damage caused by exposure to extreme wind velocity going undetected prior to flight, especially on large transport aircraft, is further discussed in: Flight Control Protection from Damage by Strong Winds.It is clearly essential that all flight control locks are removed / disengaged before flight. Failure to do so could make it impossible to initiate rotation or to control the aircraft once in flight. Control locks which are fitted or engaged in the flight deck should normally be removed before starting engines and on all aircraft, confirmation of full and free flight control movement should be made before starting a takeoff.The following examples of gust locks are from a Hawker 800:

                                                            

      Aileron/elevator locking mechanism (essentially a clamp to hold the yoke)


                                                               

                       Rudder lock (a bar that secures the pedals from moving)

 

 

 

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