CGS Ground School Technical The Air Speed Indicator
- Slides: 8
CGS Ground School Technical The Air Speed Indicator (ASI) © Crown Copyright 2011 No Part of this presentation may be reproduced without the permission of the issuing authority. The views expressed in this presentation do not necessarily reflect the views or policy of the MOD.
THE AIR SPEED INDICATOR The air speed indicator consists of: A flexible capsule, connected to the pitot source. The surrounding space is connected to the static source. The flexible capsule is connected, via appropriate gearing to a pointer, which moves over a calibrated scale. KNOTS 0 STATIC PITOT 20 30 40 50 60
THE AIR SPEED INDICATOR The Pitot pressure inside the flexible capsule consists of both Dynamic and Static pressure. The Static pressure inside the flexible capsule is cancelled out by the Static pressure outside of the capsule. This means the pointer is only affected by Dynamic pressure. KNOTS 0 STATIC + DYNAMIC 20 30 40 50 60
THE AIR SPEED INDICATOR As the aircraft accelerates the Dynamic pressure increases and the flexible capsule expands. KNOTS 0 STATIC + DYNAMIC 20 30 40 50 60
THE AIR SPEED INDICATOR Blockages If the pitot source becomes blocked, the ASI will simply not respond to changes in airspeed. If the static source becomes blocked, the static pressure component of the pitot pressure will still change with height. As the aircraft climbs and the static pressure component in the flexible capsule reduces causing it to contract and the ASI will under read. KNOTS 0 STATIC PITOT STATIC 20 30 40 50 60
THE AIR SPEED INDICATOR Blockages at height If the static blockage occurs at height, then as the aircraft descends the static pressure component within the flexible capsule increases causing it to expand the ASI over reads. KNOTS 0 STATIC PITOT STATIC 20 30 40 50 60
THE AIR SPEED INDICATOR Colour coding The ASI is colour coded to show vital speed ranges at a glance The initial white arc indicates the range 0 kts to 1. 1 times the basic stalling speed. The green arc indicates the range from 1. 1 times basic stalling speed (the lowest useful safe flying speed) to Maximum manoeuvering speed (Vman). The yellow arc indicates the range from Vman to Velocity never exceed (VNE). In this range it is possible to overstress the aircraft. Therefore control movements must be smooth and progressive and not exceed one third maximum deflection. VNE is marked with a red line. The yellow triangle indicates basic stalling speed times 1. 3. This is the manufacturers minimum recommended approach speed.
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