help me to understand IMC flying
 

On another place, in a thread about the unfortunate pilot who crashed near to the M40 yesterday, some of the responders have mentioned "FAA PTS limits are 3/4 scale deflection and +/- 10kts" wrt the conditions yesterday. Can someone explain, for me as a NPPL(M) nice weather pilot, what is the "scale deflection"? It sounds like a reading of some sort of gauge.

Ta.

That might refer to localiser needle deviation across the available scale...?

Localiser (horizontal) and glideslope (vertical) needles (or other display) for ILS (instrument landing system). If the needle gets more than half way across the dial from the centre you go around (maybe different measures for different jurisdictions, types of rating and height).

Agree. To expand a little on that, the ILS (Instrument Landing System) creates a "cone" of protected airspace from the final approach fix (center base of the cone, typically on the centreline, 2000 feet above and 6.2 nm laterally from the touchdown point) to the touchdown point on the runway (tip of the cone). As long as you're inside this cone (and above the decision altitude), you're protected from flying into the ground or other obstacles.

The ILS gauge in the cockpit shows you where you are relative to the center of the cone. If the deflection is too high, you run the risk of flying outside the protective cone. Which is bad. So you go around.

I agree with all other replies. But I am not sure anyone explained what this "3/4 scale deflection" means. As what you quote seem like limits, I will attempt to explain it in as easy a way as I can.

Talking about lateral distance... When your CDI/OBS/HSI is tuned to (and identified) a VOR station, you can see 5 dots on either side of the centreline of the instrument. Each dot represents 2 degrees. So if I am tracking the 240 degrees TO the VOR, then if my needle is on the second dot to the right of the centreline of the instrument, then that means that I am 4 degrees to the left of the desired track. (Hence I must turn right to regain track).

For you to understand the scale, full scale deflection whilst tracking a VOR means that I am 10 degrees or more off the desired track (as the instrument has 5 dots either side, each dot is 2 degrees hence 10 degrees [or more]). Half scale deflection means that I am 5 degrees off course (or 2.5 dots either side). Hence 3/4 scale deflection would be 7.5 degrees off course (or 3.75 dots either side).

I used the VOR because the scale remains constant, and it was an easy way to describe the term "scale deflection".

The term 3/4 scale deflection for VOR tracking means that at all times during your tracking, you must not stray more than 3.75 dots either side of the centreline. (meaning no more than 7.5 degrees off course)

Should you be doing an ILS, as explained above, the instrument abides by a cone, so the closer you get to the ILS beacon - the smaller the scale (hence the smaller the movement required to regain / lose track). This is true for both vertical guidance (glideslope) and lateral guidance (localiser).

So the term 3/4 scale deflection for an ILS means that at all times during the approach procedure, the needles must not exceed 3.75 dots (or 75% of the available space) on both lateral and vertical guidance needles.

EASA allows only for half scale deflections for ILS approach. If you're overshooting your glideslope in IMC conditions, you can actually get "established" on a side lobe.. if a pilot doesn't recognize that he's on a sidelobe (required rate of descent is much steeper) you might as well end up in a tragedy.. therefore nice clean intercepts are critical

This explains it pretty well.

ILS Navigation part 1: The Localizer [Aviation Theory] - Flightsim Aviation Zone

martin 123 I do not understand the term you use "overshooting your glideslope".....if you are able to explain further I request you to do so.

When operating machinery, you have to keep within certain limits. Even when visual, you do your circuit at a certain height. Why? It keeps you clear of houses and obstacles yet at the same time close the airport. If you flew your circuit 1,000' too low, what would happen? Now accepting that 100% accuracy cannot be achieved, what variation in your circuit height/altitude would be considered acceptable? We will call this variation "scale deflection"; it's not correct but it makes the point. When flying on the clocks, each and every one has to stay within certain limits. Plus or minus x knots, feet, RPM, fpm etc. Fly outside the limits and you may bump into the ground (because you can't see where it is).

PM

lots of good info, thanks. I didn't think that the comments about scales and heights made sense with any of the basic instruments I am familiar with. I was wondering if it was some sort of visibility measuring equipment, possibly the gear that gives the viz readings on automated METARs. ILS is a whole different world for me.

have a look at the image shown in What is the behavior of false glideslope signals?
When you come in way too high (overshooting the normal 3° glideslope) you can end up in a seemingly reliable GS signal on the 9° 'ghost' glideslope.

That diagram seems wrong. wouldn't you end up with a valid but upside down glide slope indication on the 6° slope too (you still have a mix of 90 and 150 Hz signals, just upside down. Cannot see how you get a null signal as suggested.

I think it depends on the equipment and your approach angle. If I remember correctly the top lobe for glideslope goes up to about 5.25°. The bottom lobe for the false glide slope should start at about 7.65°.. that means if you are approaching with 6° you should get a null.. that's the theory bit anyway. From the practical side, the closer you get to the station, the likelier you will receive the reversed indication. I haven't seen a reversed indication myself, but I heard they do exist. I reckon that's where the airports equipment category/class comes into play

A small 'amplification' to Alex90's post. An ILS installation is not a 'beacon' in the same sense as VOR or NDB. There are two antennae - a glide slope antenna on the side of the runway, adjacent to the touchdown zone and the localizer antenna which is at the far end of the runway.

By the time you are crossing the threshold, the glide slope is much more sensitive to deviations than the localizer, because the glide slope antenna is only about 1000' away, whereas the localizer antenna is still a mile or more away.

overshooting the glideslope....?
 

Thank you for subsequent explanations ref 'overshooting the glideslope'. In dim and distant past I was IF qualified pilot, in fact instructor in same......do not remember any difficulties in this regard. I think that if the pilot is flying either an approved approach procedure or simply flying vectors and altitudes as advised by ATC with view to achieve ILS intercept......the airplane will not 'overshoot glideslope'.

... or if not, eg a self-positioned practice approach in VMC on the way back from a bimble, then this does have to be taken into account ...