This week included work on the instrument landing system (ILS), an approach method that is considered as 'precision'. It is deemed as more precise as the pilot now has guidance in both azimuth (direction) and elevation (level), and is the principle instrument approach used at most commercial aerodromes. I was looking forward to this one the most as it is the type of approach I will fly for the majority of my career, so the simulator was positioned at Auckland to give it a go.
As previously mentioned the approaches we had worked through were known as 'non-precision'; effectively directional guidance is given to get you to the aerodrome itself, and the pilot must use the advisories on the plate to try and achieve the planned descent profile. These types of approaches can be flown very accurately, but the margin for error is a lot larger than with the ILS, and therefore additional safety factors are built in. The main one is the Minimum descent altitude (MDA); this level is indicated on the plate, and should be seen as the 'descent floor' for the procedure, below which the aircraft must not descend unless they are visual with the runway. The aircraft can fly level at the MDA in the hope of getting visual with the runway, but the longer this takes the steeper the final descent would have to be in order to complete the landing. In the case of Hamilton this is over 500 feet which puts the aircraft about a mile from the airfield, so when there is a combination of poor visibility and low cloud, the pilot is unlikely to complete the approach.
In comparison the ILS has a 'decision altitude' (DA), a point where the pilot looks out the window to see if they are visual and can continue the approach. In the case of the ILS the aircraft can legally descend below this level, but if a missed approach is to be executed this will have been commenced as the aircraft passes the DA. This isn't as critical as the non-precision approach, as the aircraft would still be receiving track and level guidance, and as long as the aircraft wasn't dangerously low already it won't hit anything. This is prevented with the rule that if there is more than "half scale deflection" on the glideslope indicator, a missed approach must commence.
This is the primary flight display from an Airbus instrument panel. In the centre is the artificial horizon (AH), the airspeed indicator is on the left (ASI), direction indicator at the bottom (DI), and the altimeter on the right. Between the AH and the altimeter there is a magenta diamond with four dots and a line. This is the glideslope indicator, the top and bottom dots being full scale deflection and the line being the centre. To fly the ideal approach the pilot will plan a rate of descent, then with minor adjustments they should be able to keep the diamond on the line, which will move to indicate what action is required from the pilot. In the case shown the diamond is indicating 'fly up', but this is due to the aircraft intercepting the glideslope from below so there is no issue. If however they get this indication once they had already intercepted the glideslope once, there would be a consideration that a missed approach must follow if the situation isn't rectified.
For the moment these types of approaches are busy, but certainly manageable as the aircraft is easy to manipulate. When we get into the Airbus however the aircraft will have so much more inertia and is going considerably quicker, so accuracy is key to prevent any errors snowballing. Next up we've got more approaches and then we should be moving onto GPS routes in readiness for our last 5 flights in the Cessna.
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