The Trap in the MAP on ILS
Some operators require both pilots to stay on MAP mode (B737 Classics) during an ILS because the localiser and glide slope displacement is readily seen on the ADI. Others allow a choice between HSI mode and MAP mode.
On ILS with flight director, we know that the ADI localizer automatically goes to expanded scale. That's fine. Now try this in the simulator: Set up the ILS as above but on MAP mode. Ensure IMC.
Near the outer marker, deliberately turn the aircraft say 10 degrees off track (to simulate incompetent handling) and note that very quickly the expanded mode needle on the ADI goes to full deflection and off scale. Stay on glide slope while doing this.
Beyond full scale ADI expanded localiser you now have no idea how far you really are off the centre line. Only HSI mode will show you that. Looking at the MAP mode will not tell you how many dots you are off centre.
After several seconds of flying this offset heading from the ILS centre line, freeze the simulator and switch from MAP to HSI mode. By now after say 20 seconds you will see that the aircraft is beyond full scale normal localiser and heading for potential disaster depending on terrain.
Now switch the simulator to CAVOK and note the runway way off to the left or right although you are still on GS.
What is my point, you are entitled to ask. It is this.
If you inadvertently deviate from the ILS centre line for whatever reason (drift or inaccurate flying) then once the expanded localiser reaches full scale for just a few precious seconds you lose awareness of just how far you have deviated unless you quickly switch to HSI mode and check the number of dots deviation.
You can go completely off the HSI localiser scale -yet the expanded localiser on the ADI will still stay expanded and all you know is that you are currently more than just one dot off centre. In other words the expanded localiser needle on the ADI is now useless.
The MAP mode certainly may aid situational awareness in terms of positions of the runway centre line and of other information such as missed approach track, but there is no way you can judge the number of dots off track merely by scanning the MAP.
And it is the number of dots off track that dictate whether or not the deviation is serious enough for an immediate go-around. And only the HSI mode will give you that information. For this reason I believe that for an ILS the HSI mode is safer in the long run than MAP. Having said that I am aware that Boeing recommend MAP mode for an ILS.
And hands up all those who have never deviated badly off the ILS centre line on an ILS?