View Full Version : The Trap in the MAP on ILS

1st Jun 2003, 21:48
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?

2nd Jun 2003, 02:50

For those of us unfamiliar with the aircraft or display setup, could you indicate:

1) lateral distance or angular displacement indicated by FSD on the ADI

2) lateral distance or angular displacement indicated by FSD on the HSI, and by one dot on the HSI

3) the scale of the map as depicted in MAP mode



2nd Jun 2003, 04:33
I had always thought it was necessary (and prudent) to use raw data for the capture. However, during my last CQ training, I was instructed it was not necessary as the expanded mode provides sufficient information (and Progress page 2 gives offset from an FMS calculation). Since company standards don't specify what mode to be in for ILS approaches, and after thinking about your post, it may not be a bad idea to remain in raw data.

2nd Jun 2003, 08:51
I thought the 737 classic had steam driven dials. Which ancient 737 are you talking about?

2nd Jun 2003, 12:19
"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. "

Is your F/O also situationally unaware? :D

Is this typical display behaviour? Do most glass cockpit aircraft types latch the MAP displays in the expanded mode, or do the displays go back to normal if you go out of range of the expanded display? (say, 1.25 dots)

I know there are a few differences when it comes to this sort of thing. As far as I know, the 747-400, for example, only produces an expanded display when the LOC is engaged (so there will be less chance of you going off the ILS beam, once established thereon).


2nd Jun 2003, 17:47
Stillalbatros, 737's are called "original" = 100 & 200
"classic" = 300, 400 & 500, with ADI - MAP EFIS
"NG" full EFIS.

In the 744 precision simulator at least, if you deviate too much, the scale turns into normal again.

My brother's company sop calls for the opposite as aerobat's one.

2nd Jun 2003, 22:01
Bookworm. Not quite sure what you mean by the FSD but assume it is the localiser and glide slope info on the ADI and HSI?
Normal LOC deviation is one degree per dot. When VOR/LOC engaged (not necessarily on auto pilot either) the ADI scale automatically expands to indicate half a degree deviation per dot. It remains expanded until after landing roll-out or on a go-around with RA greater than 200ft.

There is no distance lateral deviation indication on either the expanded ADI scale or HSI although you could work it out by mental arithmatic if you knew the exact distance from the localiser aerials. Forget that for a joke. But as an example if you are 5.486 miles from the threshold then one dot on the normal scale on the HSI means you are 774 feet off centre line.

If you fly the ILS without the flight director then the ADI localiser scale is the same as the HSI - that is no expansion. In that case if you are on MAP mode for situational awareness it doesn't really matter because the ADI localiser gives you all the information necessary to tell you exactly how many dots you are off the centre line.

Put another way - if you allow the aircraft to stray off the centreline full scale on normal HSI mode then it is an immediate go-around situation. But if you are on MAP mode and relying on the expanded scale on the ADI to keep you within limits when on flight director, then you are whistling into the wind

Of course a brilliant PNF would quickly advise the PF that he had deviated off scale on the expanded ADI when on flight director coupled to ILS. But the moment that the PF allows the aircraft to drift off beyond full scale on the expanded localiser you are rapidly going into no-man's land especially if you are maintaining the glide slope.

In the simulator we were unpleasantly surprised how quickly the expanded localiser moved to off limits while at the same time we were unable to detect a significant lateral displacement while staring at the MAP mode even on the 10 mile scale.

At no stage did the expanded scale automatically revert to normal scale even though we had reached beyond full scale displacement on the HSI normal scale. I believe a safer design philosophy would be to have automatic reversion from expanded scale on the ADI to normal four dots if a significant lateral deviation occurred.

3rd Jun 2003, 00:04
Thanks for the extra info Hudson and sorry to have confused: FSD is Full Scale Deflection, but you gave the details I was looking for anyway.

5th Jun 2003, 06:24
I find it hard to imagine why one would drift so far of the localiser when using a flight director coupled to it.

The fact that flight directors can be so compelling has drawn many people into following the FD indications despite the instructions being in error.

What is the smallest scale on your MAP display?

5th Jun 2003, 10:58
"I find it hard to imagine why one would drift so far of the localiser when using a flight director coupled to it."

If it happens just one time, and being in raw data helps you catch it......

It would be unlikely to see such an event in the typical intercept scenario. But what about a busy approach controller trying to squeeze you in (and you accept it), a busy cockpit (late accomplishing checks), mountain environment, autopilot on, approach gives you an intercept heading but the bank selector is in 10 degrees instead of auto (you shoot through the LOC), it's 0800 but your body thinks it's 0200 (flew through the night), ....

seat 0A
5th Jun 2003, 15:45
The simple answer is: just go around!
Once you are in the situation where you are looking at a full scale deviation in the expanded mode, you are outside the safe flightpath.
Even if you would have had the raw data display, you would still be in the same place. In IMC there is only one option: go around!
Flying in the expanded mode also gives you the advantage of the deviation warning.

Trying to recover from a position that far from the correct one, in IMC is not a good decision. Just take your losses and try again fresh.

5th Jun 2003, 23:08
SeatOA. Interesting re LLZ deviation warning. For some reason it did not actuate in our 737-300 simulator. Must have another look next time. Re go-around if you are one dot on expanded scale. At this point the HSI will indicate less than half scale off centre line which is no cause for a GA yet.

By the way the MAP is on 10 mile scale.

6th Jun 2003, 10:18
I vote for MAP and raw data on the ADI.

If you line up on final and your raw data will confirm/deny the validity of your MAP display. If your MAP is good then you have much more situation awareness. Ever had your ILS reciever fail on you with no flags? It happens. I had it once. Ever had a false capture, or a false GS lock on? How can you tell if you don't have MAP mode up? Having the MAP mode up will save your situational awareness and help you make better decisions.

6th Jun 2003, 17:44
Map display cannot be an official means to verify your LOC and Glide Slope.
You must verify the LOC with an NDB or VOR radial, and Glide Slope with a DME or LOM (or, as a last resort, an ATC radar position).
Once you have done so, I agree a MAP display is better.
There can't be an absolute truth, the important thing is to make a good valid check.
Display mode preferences depend on how your company believes you are gonna tend to make these checks: if they think you will be lulled into a false sense of security by a MAP display, they might impose you the use of EXP VOR/ILS.
I would leave the choice to the captain, but ground him if he fails to check his Glide Slope by the correct method.

7th Jun 2003, 12:13

Is this a ETA course?, and is the sim VH-CZA (ex AN) or TJxx (current QF 300/400 machine???????)

Also depends on the modelling on the sim computer!

8th Jun 2003, 21:11
Map shift and ILS approaches. Classic 737
All EFIS classic drivers flying into a remote strip with a u/s VOR but serviceable ILS know the FMS updates it’s “lateral” position once on the localiser. The map shift can be a few miles.

Once upon a time….. we had a false localiser capture a full three miles off the centreline. I picked it up because
1. It happened sooner than I expected (after manoeuvring around weather tru the localiser and back towards it expecting to become visual at 3500 AGL.)
2. The track that was 212 instead of the required 222 and the NDB showed us off track as well
3. On our request, Radar confirmed the three miles right of centreline.
As we began to “go raw data” on the NDB to correct we became visual, the FO corrected visually whilst I had a close look at the EFIS.

Except for the giveaways above, all indications on the EFIS (on 10 mile scale) were that we were on centreline with the runway ahead. Only a very close look showed the white track line on 212 missing the runway
I selected IRS 1 on the position page and entered, the map adjusted, putting the runway where it actually was, to the left. As I watched the map updated itself on the false localiser and put the runway dead ahead again!.

As we approached the real centreline, the localiser needle twitched, went full scale and then centred as we intercepted the real centreline. At this stage the “map” runway was now off to the right. A few seconds later the FMS updated itself on this new localiser and put the map in front again.

I recount this little tale to explain why I always check all available raw data before and whilst using the wonderful map mode.
If at any time the localiser pointer moves off the ADI expanded scale I immediately select VOR/ILS on the EHSI.
I now fly both classic and the NG which of course has GPS and map shift does not happen on the NG.

9th Jun 2003, 00:23
All EFIS classic drivers flying into a remote strip with a u/s VOR but serviceable ILS know the FMS updates it’s “lateral” position once on the localiser.
Sadly not QUITE true, gg! Good ol' BA appear to have taken the feature OUT of their 'BASpec' 737-400s:{