SD3-60 AIRCRAFT MAINTENANCE MANUAL

Transcription

1 AMM AUTO-FEATHERING SYSTEM - DESCRIPTION & OPERATION 1. Description A. General This system is employed to speedily reduce propeller drag by automatically feathering the propeller on an engine which sustains a substantial loss of torque when both engines are operating in the take-off range (power levers set for gas generator speeds at or in excess of 88% Ng). NOTE: (1) The system is independent of the manual feathering controls (PROP levers, Chap. 76). (2) When operationally functioned for a failing engine, interface circuitry will cause an OFF selection for that engines intake anti-ice vane to be overidden; this will further diminish power plant drag. Refer to , pb1. Control circuitry (para. B) is such that when comissioned for a failing engine, the autofeathering facility for the opposite (running) engine is inhibited. Auto-feathering action for each engine is identical, being initiated by a solenoid valve on the propeller overspeed governor. When energised, the valve opens to dump propeller oil to the sump, allowing the propeller blades to feather under the action of springs and counterweights. The system is entirely automatic, requiring only the availability of the 28V dc operating supplies; for the left and right engines, these are respectively taken from the associated essential services busbars via 5 amp C/B Nos. 20 and 170 on distribution panels 1D and 2D. A test facilty permits check of the auto feather valve functions at lower power settings. On aircraft incorporating Serv. Bull. SD (Mod A8575), auto-feathering is disarmed when the landing gear is selected down on approach. The associated inhibit circuitry is described in Para. C. B. Circuitry Refer to Figure 1. The 28V dc supplies to the solenoid valves are in each case controlled by two microswitches in series, one operated by each engine power lever. With both levers advanced to positions relative to at least 88% Ng, the system is armed and is under the control of four pressure switches, two on each engine. When an engine fails and its torquemeter pressure falls, the contacts of one of its pressure switches open at 12 psi to break the circuit to the solenoid valve on the other engine, thus inhibiting the autofeather function for that engine. A continuing fall in torquementer pressure on the failing engine will cause its other pressure switch to operate at 8 psi to complete the supply to its solenoid valve via the closed contacts on the 12 psi pressure switch on the opposite engine. Page 1

2 System indication is afforded by two modules located centrally at the top of panel 1P; when illuminated, each displays the legend AUTO-FEATHER ARM. Indication appropriate to system condition is as follows: (1) Power levers below 88% Ng: Both 'out' (2) Power levers at or above 88% Ng, engine operating normally: (3) Power levers at or above 88% Ng, left or right engine failing: Both AUTO-FEATHER ARM captions lit (each being supplied through series connected high pressure contacts on both 12 psi switches, left and right). Both AUTO-FEATHER ARM captions go 'out' when fall in torquemeter pressure causes failing engines 12 psi switch to open its high pressure contacts. Two test switches (ganged) on panel 11P permit test functioning of the autofeather valves at lower power settings (when held at the TEST position, the engine power lever microswitches are short-circuited). C. Auto-feather disarm on approach circuitry (Post Serv. Bull. SD ). An auto-feather inhibit relay is introduced in the landing gear control and indication circuitry. Refer to Figure 2. The modified circuitry does not impede ground testing or normal arming/ activation of the auto-feather system on the ground, during take-off or en route operations. Initial energising of the inhibit relay occurs when the aircraft is resting on wheels and the essential services busbars are energised. In this condition, contacts on the de-energised weight switch relay, and the energised downlock engaged relay, series connect to the coil of the inhibit relay. The relay will then lock on through the DOWN contacts of the landing gear selector switch. Power for arming/activation of the auto-feather system is then availed through closed contacts on the energised inhibit relay. When the landing gear is selected UP after take-off, the inhibit relay de-energises (lock on circuit through selector switch is broken). In this condition, the auto-feather system will remain armed (powered direct from the UP contacts of the landing gear selector switch, bypassing the inhibit relay). On approach, when the landing gear is selected DOWN, the inhibit relay remains deenergised, thus isolating the power supply to the auto-feather system. After landing, the weight switch relay will de-energise and cause the inhibit relay to re-energise; this restores the auto-feather system to a normal condition i.e. permit automatic arming for the next takeoff. Page 2

3 Autofeathering System Figure 1 Page 3

4 2. Operation A. System System operation is as detailed in the preceding paragraphs, summaried as follows:- With engines running, advancement of both power levers to positions relative to at least 88% Ng will arm the system. In this condition (a) all four torque pressure switches, two left and two right, will be held on their 'high pressure' contacts and (b) both AUTO-FEATHER ARM captions on panel 1P will be lit (each being supplied through the associated series-connected power lever switches and the appropriate high pressure contacts on the 12 psi switches, left and right. Subsequent failure of either engine will initially cause its 12 psi switch (opposite engine inhibit) to make its low pressure contacts on a falling torquemeter pressure of 12 psi (equivalent to 1000 ft. lb.). This will cause both AUTO-FEATHER ARM captions to go out and will disarm the opposite engines autofeather function. Continuing fall in torquemeter pressure causes the engines second pressure switch low pressure contacts to make at 8 psi (equivalent to 670 lb.ft.); this connects the related supply to the auto-feather valve. The valve thus energised, opens to vent propeller hub oil to sump, allowing the blades to feather under spring and counterweight action. B. Testing Both engines auto-feather valve functions are normally checked simultaneously at low power settings, employed the ganged AUTO-FEATHER test switch is subsequently described; however, if one engine is static (not under test), it is recommended (though not essential) that its associated auto-feather C/B is tripped). NOTE: Propeller on a static engine would already be feathered and what could only be an aural check of its auto-feather valve function would be inconclusive as a maintenance check. When the engine/s are running, and power levers set at FLIGHT IDLE, all four auto-feather pressure switches (two left, two right) will be on their low pressure contacts. Operating supplies to the auto-feather valves in this condition are directly made available by holding the AUTO-FEATHER test switches at TEST and checking that:- (a) propeller prpm decreases significantly and hunts (related valve energised open). (b) the engine associated AUTO-FEATHER ARM caption is lit. Upon release of the test switches; the indicator caption/s go out and confirmation that the auto-feather valves have de-energised shut is provided by observed resumption of pre-test speed. C. Auto-feather disarm on approach circuitry (Post Serv. Bull. SD ). Operation of the subject circuitry is described in Para. 1Refer to C.. Page 4

5 Auto-feather Disarm on Approach Circuit (Post Serv. Bull. SD ) Figure 2 Page 5

6 AMM AUTO-FEATHERING SYSTEM - MAINTENANCE PRACTICES 1. Adjustment/Test A. Function test the Autofeather System Refer to , pb1. This test is detailed in System Checks (engines running) and includes check of the intake anti-icing vane automatic deployment function (drag limiting). Refer to , pb201. Page 201