In the USA, AIM 5-5-5 paragraph a1 specifies when a missed approach is required. I have also heard that a missed approach is required if the aircraft reaches full scale CDI deflection on a localizer course (this old CFI test standards doc from 2001 says it’s recommended in paragraph b).

Where is this requirement written? Is it true for all types of approaches, or only ILS and LOC?


The point being, once you've reached a full scale deviation in IMC, there's no telling how much beyond it the airplane has gone. Your USA AIM 5-5-5 is quoted here, and point 'b' is sufficient to conclude an immediate Missed Approach:

5-5-5. Missed Approach
a. Pilot.
1. Executes a missed approach when one of the following conditions exist:

  • (b) Determines that a safe approach or landing is not possible.

With a full scale deviation and the LOC beam narrowing as you approach the RWY, what is the pilot going to steer at low altitude and descending lower in IMC? What is the IFR reference that the pilot would use? A VOR course? An FMC course? A locator beacon (NDB)? Even if these existed, there would have to be an IFR procedure to continue to landing, including revised minimums.

Quoted below are the Elements of a Stabilised Approach, widely accepted as The Gospel by regulators and industry, All manufacturers have incorporated into their procedures, this concept from the Flight Safety Foundation's ALAR 'toolkit':
(ALAR = Approach and Landing Accident Reduction )
(notice too, the applicability to VMC conditions)

Most airlines and other aviation organisations specify minimum acceptable criteria for the continuation of an approach to land. These vary in detail but the following summary published by the Flight Safety Foundation is one view of the important considerations.

Their Approach-and-landing Accident Reduction (ALAR) Briefing Note 7-1 suggests that "all flights must be stabilised by 1000 feet above airport elevation in IMC and 500 feet above airport elevation in VMC. An approach is stabilised when all of the following criteria are met:

  • The aircraft is on the correct flight path
  • Only small changes in heading/pitch are necessary to maintain the correct flight path
  • The airspeed is not more than VREF + 20kts indicated speed and not less than VREF
  • The aircraft is in the correct landing configuration
  • Sink rate is no greater than 1000 feet/minute; if an approach requires a sink rate greater than 1000 feet/minute a special briefing should be conducted
  • Power setting is appropriate for the aircraft configuration and is not below the minimum power for the approach as defined by the operating manual
  • All briefings and checklists have been conducted
  • Specific types of approach are stabilized if they also fulfil the following:
  • ILS approaches must be flown within one dot of the glide-slope and localizer a Category II or III approach must be flown within the expanded localizer band
  • during a circling approach wings should be level on final when the aircraft reaches 300 feet above airport elevation; and,
  • Unique approach conditions or abnormal conditions requiring a deviation from the above elements of a stabilized approach require a special briefing.

An approach that becomes unstabilised below 1000 feet above airport elevation in IMC or 500 feet above airport elevation in VMC requires an immediate go-around.

  • $\begingroup$ This is interesting information, but it does not address whether this is a mandatory requirement by the FAA, and if so where it is written $\endgroup$ – asb1230 Jan 5 at 20:02
  • $\begingroup$ I don't think there is a law or regulation that specifies that because it's more of an airmanship issue. Operators will have a requirement in their SOPs however. This is a good example of that sort of thing taken to an extreme en.wikipedia.org/wiki/First_Air_Flight_6560. $\endgroup$ – John K Jan 5 at 20:50
  • $\begingroup$ Imagine toodling along being beyond the full deviation at low altitude, blind in IMC. As the person in-charge in the cockpit, I don't think that's an acceptable situation. $\endgroup$ – skipper44 Jan 5 at 23:18
  • $\begingroup$ Well, this would highly depend on when the CDI went to full scale deflection. I would imagine that it would not do this before the minimums. Probably not even before the VDP or even the AER. But, it definitely would for a circling approach. But, that is academic and does not address the actual question posed. $\endgroup$ – Dean F. Jan 6 at 3:52
  • $\begingroup$ Ironically, I'd edited out the following to make the answer less complex: "Making it more difficult to estimate the deviation is the fact that ILS LOC full scale is normally referenced to the lateral edges of the RWY at the landing Threshold. Thus it varies with RWY length ie, a long RWY has a narrower LOC beam than a short RWY." $\endgroup$ – skipper44 Jan 6 at 4:51

I agree with skipper44's interpretation of the maneuvers for a safe landing being the broad reasoning that if you dont know how far off course you are you cant make a safe landing. I did some more digging and Ill present another option from the FAA that might also satisfy the question.

According to 14 CFR § 91.181 - Course to be flown when operating under IFR you need to be "on course"

§ 91.181 Course to be flown. Unless otherwise authorized by ATC, no person may operate an aircraft within controlled airspace under IFR except as follows:

(a) On an ATS route, along the centerline of that airway.

(b) On any other route, along the direct course between the navigational aids or fixes defining that route. However, this section does not prohibit maneuvering the aircraft to pass well clear of other air traffic or the maneuvering of the aircraft in VFR conditions to clear the intended flight path both before and during climb or descent.

Now if we dig into order 6750-16E which lays out the spec for designing ILS systems we can see that the all CDI's must be calibrated to 150 micro amps equals a full deflection and likewise this needs to be calibrated to the edge of the ILS course (bolded for emphasis)

c. Localizer Receiving Equipment. The localizer receiving equipment generates a meter deflection indication, which corresponds to differences in depth of modulation. When the aircraft is on course there will be a zero-centered meter. When the aircraft is left side of the course, the Course Deviation Indicator (CDI) needle will deflect to the right and when the aircraft is right side of the course, the needle deflects to the left. The CDI is calibrated so that 150 micro amps (.155DDM) represent full-scale deflection on each side of the course centerline. The angular value corresponding to the 150-0-150- micro amp values is defined as the course width. The standard or “tailored” course width provides for 700 feet between the full-scale deflection points at the runway threshold. When the localizer antenna to threshold distance is less than 6678 feet, the course width is capped at a maximum of 6.0 degree. Localizers used for Category I service only may be “non-tailored” if necessary within the limits of 6.0 degrees maximum and 400 feet at threshold minimum. Localizers supporting Category II/III service must be “tailored.”

As such I would say that if your needle goes full deflection you are no longer on course and thus no longer legal under 91.181 and dont know how far you are from the course anymore so you have no way to know how to intercept it (even if you practically know where it is) but going missed allows you to easily establish yourself on the missed course (if you have two CDI's your likely tuned in already anyway or should be)

  • $\begingroup$ Interesting info on localizer equipment. FAR 91.181 is interesting... victor airways for instance are 8 miles wide. If I’m not on the centerline of the airway but a mile to one side, am I operating illegally? $\endgroup$ – asb1230 Jan 7 at 16:39
  • $\begingroup$ It'd be interesting to know how to maintain a lateral (say 1 nm) deviation from a "victor" airway centerline using VOR radials? A victor-AWY segment vs descent close to the ground on a narrowing ILS beam is not the best comparison. However, the 8 mile figure says it all, 1 nm off the required Track would not be a violation. But, if you do not, or cannot return to the radial, it would warrant giving a shout to ATC. Modern PBN routes, very explicitly define their 95% containment figures as RNP. A 1nm deviation would not be illegal on a RNP1 ARR, but it would be on an RNP 0.3nm Approach. $\endgroup$ – skipper44 Jan 7 at 18:20

The FAA Airman Certification Standards (available here) for the instrument rating require the following:

"The standard is to allow no more than a 3⁄4 scale deflection of either the vertical or lateral deviation indications during the final approach."

From a purely legalistic point of view, not performing to this standard would invalidate an instrument rating.

But far more practically: it's dangerous! Radio beams are not perfectly accurate, nor are receivers.

For comparison, the UK Flight Examiner's handbook CAA link here requires applicants for commercial or higher license to not exceed half-scale deflection.

  • $\begingroup$ I'm not sure what you mean by "not performing to this standard would invalidate an instrument rating"? $\endgroup$ – Pondlife Jan 13 at 17:27
  • $\begingroup$ @Pondlife The ACS specify the skills required for the instrument rating. Full-deflection (on an instrument approach, with due regard to relevant circumstances) would not be consistent with those requirements. $\endgroup$ – Ugo Jan 14 at 11:01
  • $\begingroup$ Right, if you don't fly to ACS standards then you won't pass an instrument checkride or IPC. I guess when I hear "invalidate" it sounds more like losing your instrument rating if you don't fly to those standards in general. $\endgroup$ – Pondlife Jan 14 at 19:20
  • $\begingroup$ I believe the Standard applies regardless. On a checkride, the result might be a Notice of Disapproval. Out in real life, the result might be FAA Certificate Action. From their website: "Suspensions of indefinite duration are issued to prevent a certificate holder from exercising the privileges of a certificate pending demonstration that the certificate holder meets the standards required to hold its certificate." But this is getting very legal. I think the original poster was looking for information useful in decision-making. ;-) $\endgroup$ – Ugo Jan 15 at 12:04

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