False Localizer Course Captures in Autoflight


by Mark Bucken, Specialist, Airspace and Service Requirements, NAV CANADA

“What’s it doing now?” More than a few pilots have uttered this phrase while trying to figure out why their autopilot was doing something unexpected. This is why it is of critical importance that cockpit crews maintain situational awareness and monitor what the aircraft is doing at all times, especially when in autoflight.

There have been recent reports of aircraft arriving at Winnipeg, Man., mainly from the east, experiencing false localizer course captures while on autopilot. The problem usually occurs while the aircraft is either on a standard terminal arrival (STAR), or after it has been cleared for the visual approach to Runway 36.

It appears that after the aircraft has been cleared for the approach, the crew, anticipating flying the instrument landing system (ILS) Runway 36 approach, are using either the autopilot NAVIGATION (NAV) on the STAR or HEADING (HDG) mode to position for intercept of the localizer.

On occasion, pilots have then prematurely selected LOCALIZER or APPROACH (APR) mode, anticipating that the flight director will maintain the present heading to intercept and subsequently capture the localizer.

Unfortunately, the early arming of the APPROACH mode allows the autopilot to initiate a turn to track the inbound course when it senses an early fluctuation in the localizer signal. It should be noted that in all of these occurrences, the crews immediately detected the flight deviation and corrective action was taken.

Section COM 3.13 of the Transport Canada Aeronautical Information Manual (TC AIM) provides a great deal of guidance on localizer coverage volume, localizer signal limitations, and cautions about their usage.

The coverage and validity of ILS localizer signals is within 35° of either side of a front- or back-course nominal approach path to a distance of 10 NM, and within 10° of either side of a front- or back-course nominal approach path to a distance of 18 NM (see  Figure 1). Signal variations outside of these sectors are known to create false capture conditions that satisfy the automatic flight control system’s localizer capture logic.

Figure 1
Figure 1

Click on image to enlarge.

Figure 2: Overlay of the Generic Localizer Coverage
on the ILS Runway 36 at Winnipeg airport.
Figure 2: Overlay of the Generic Localizer Coverage
on the ILS Runway 36 at Winnipeg airport.

Click on image to enlarge.

Figure 2 provides an example of what the generic localizer area would look like at Winnipeg for the approach on Runway 36, based on Figure 1.

Through the process of recurrent flight inspections, no problems with front- and back-course (if published) have been observed within the published angles based on the course centreline. The arming of the approach outside the published sector could result in a premature indication equivalent to an approaching or intercepting on-course centreline. Some flight guidance systems are more sensitive to these fluctuating localizer signals than others and may therefore initiate an early turn in an attempt to intercept the approaching localizer centreline.

With the airplane turning to acquire the “captured” inbound course, just like a normal localizer capture, the first indication of a problem would typically be the localizer deviation displayed on the attitude direction indicator (ADI)/primary flight display (PFD).

Another indication of a problem could occur when the horizontal situation indicator (HSI)/navigation display (ND) ILS becomes erratic or maintains a continuous offset with corresponding unpredictable autopilot control or flight director guidance.

In order to minimize the possibility of a false localizer course capture during an ILS approach, crews should use raw data sources to ensure and verify that the aircraft is on the correct localizer course prior to initiating an auto or coupled approach.

The following cockpit procedures are recommended in the TC AIM (COM 3.13.1 (c)):

  1. APPROACH MODE should not be selected until the aircraft is within 18 NM of the threshold and the aircraft is positioned within 8˚ of the inbound ILS course; and

  2. pilots should:

    1. ensure that the ADF bearing (associated with the appropriate NDB site) is monitored for correct runway orientation;

    2. be aware when the raw data indicates that the aircraft is approaching and established on the correct course; and

    3. be aware that, should a false course capture occur, it may be necessary to deselect and re-arm the APPROACH MODE in order to achieve a successful coupled approach on the correct localizer cours

In other words, a coupled approach should be closely monitored, including referring to any other bearing sources, to ensure the aircraft is established on the localizer centerline before commencing final descent.

Whenever flight crews experience false localizer signals, they should report them to ATC for follow-up to determine if the ILS is operating within specifications.

Canadian Owners and Pilots Association

COPA Corner: The Fix is Only as Good as the Write-up

The following article was published in the April 2009 issue of COPA Flight under the “Pilot’s Primer” column and is reprinted with permission.

If you fly in an organization with a fleet of aircraft, it’s likely that you have a procedure for writing up squawks on them when something goes wrong.

Flying clubs, partnerships, flying schools, and commercial organizations alike find it not only convenient, but also effective to have some sort of system for recording aircraft squawks so that maintenance and the next pilot(s) are aware of the problem.

The squawks are read by each pilot (ideally) prior to flight as part of the pre-flight preparation. A mental note of recent problems that have been repaired and an assessment of deferred squawks are made to determine if the aircraft is sufficiently airworthy for the intended mission.

Sometimes the words “could not duplicate” are written as the mechanic’s response to problems that are transient in nature, were simply imagined by the pilot, or so poorly written-up that the mechanic really had no idea what they should be troubleshooting.

Imagined problems do occur, but rarely, and are usually the result of the pilot hearing or seeing something they think they’ve never heard or seen before. Many times these “problems” are a normal part of everyday operation.

Navigation radio anomalies are frequently written-up like this as a result of pilots being unaware of ground facility anomalies that affect instrumentation. For example, tall corn crops off the end of the runway have been known to wreak havoc with the stability of the VOR [VHF omnidirectional range] signal at my home airport. That particular problem could easily be confused with a bad CDI [course deviation indicator], a problem that the avionics technician will not be able to duplicate in the shop.

In the grand scheme of things, the real problem is poorly written squawks that tell the mechanic virtually nothing about the nature of the problem, or gives them any idea where to begin troubleshooting.

We’ve all heard of some of the classic examples like, “Squeak in cockpit” to which the mechanic writes: “Cat installed.” Or how about this one: “Number three engine missing.” Mechanic’s corrective action: “Engine found on right wing after brief search.”

Regardless of the veracity of these two examples, these types of write-ups are far from adequate and usually result in a less than adequate solution from maintenance. Many times, the mechanic tasked with figuring out what is wrong cannot not properly duplicate the circumstances in order to see or hear what the pilot has written-up.

In other cases, the pilot fails to provide the exact circumstances under which the problem occurred. This is especially important with engine and avionics problems since there are so many possible reasons they could occur.

So how should a pilot write up a problem so that the mechanic has half a chance of finding a solution?

Start by simply indicating what you believe is malfunctioning. Then, indicate how it is malfunctioning. Use your senses for this part. Does it smell? If so, what does it smell like? Did you see something odd happen? Describe it so anyone can visualize it. Did you feel something? Try to describe the sensation, and do so with an explanation that the mechanic might be familiar with. For example, saying that the “Nose gear doors moan like a constipated rhinoceros” may not mean much if the mechanic has no applicable experience with rhinoceroses.

Start by simply indicating what you believe is malfunctioning. Then, indicate how it is malfunctioning.
Start by simply indicating what you believe is malfunctioning.
Then, indicate how it is malfunctioning.

Finally, describe the circumstances under which the problem occurred: phase of flight, on the ground or in the air, power settings if having engine problems, altitude and distance from NAVAIDs [navigation aids] when experiencing radio problems, etc.

If the problem was radio-related, always inform the mechanic of anything you tried to troubleshoot yourself. Also report any comments made by ATC on radio or transponder problems. Believe it or not, several squawks of the same nature in a short period of time, but on different aircraft, led our avionics technician to suspect a problem with ATC equipment. He was right! Had our people not properly written up those squawks, ATC might have been unaware of their own radio problems for some time.

Sometimes, the pilot can’t give enough information about a problem to be of assistance to the mechanic. Engine problems are perhaps one of the most serious problems mechanics deal with, and they may also be one of the hardest for pilots to write-up due to the myriad of things that could actually be wrong.

We once grounded our family airplane for severe engine roughness and vibration just prior to final descent and landing at Moline, Illinois. The mechanics diligently checked the engine and performed a run-up, finding no problems other than some fouled spark plugs.

My father and brother proceeded to test fly the aircraft only to experience the problems again shortly after takeoff. They made a quick return to land and took it back to the shop. This time, the mechanics performed a borescope on the engine and found a cylinder with excessive oil in it. Prognosis: sticking exhaust valve. A new valve and some cylinder reworking and we were set to go.

This example is a case where there is little evidence of the cause of the problem other than the sensation. RPM drop was not significant, likely due to it being a six-cylinder engine. The mechanics probably attacked the problem starting with the most likely sources, like spark plugs, magnetos, etc. Like doctors, mechanics, when dealing with unclear problems, will not automatically assume the rarer problem right from the start. Under circumstances such as these, the more information you can provide, the better.

Many problems have three or four symptoms that are the same but an additional one that distinguishes it from the rest. Having that last symptom in your write-up could be the difference between a solution and “ops chk ok, could not duplicate.”

Writing up an aircraft squawk is something of an art;  the pilot must be articulate enough to get across the true nature of what went wrong so the mechanic has the right cues in order to proceed with effective troubleshooting.

If you have difficulty putting the experience into words, the best alternative is to seek out a mechanic, or an experienced pilot at the very least, and explain your problem to him or her. For many problems, I follow up the write-up with a call to the maintenance shop when I think that they may have trouble understanding what I was  experiencing.

In doing so, you also show the maintenance personnel that you are concerned about the problem and add some ownership to the solution that might not exist from a simple impersonal write-up.

Remember as well that not all mechanics are pilots, so they may not understand a squawk written in pilotese. Get your point across in plain English, but keep it short and simple and you’re more likely to see a real corrective action.

This article was written by Donald Anders Talleur, an Assistant Chief Flight Instructor at the University of Illinois, Institute of Aviation. He holds a joint appointment with the Professional Pilot Division and Human Factors Division. He has been flying since 1984 and, in addition to flight instructing since 1990, has worked on numerous research contracts for the Federal Aviation Administration (FAA), Air Force, Navy, National Aeronautics and Space Administration (NASA), and Army. He has authored or co-authored over 180 aviation-related papers and articles and has an M.S. degree in Engineering Psychology, specializing in Aviation Human Factors.

Search and Rescue Experts Need Your Help… Register Your Beacon

by Major Clarence Rainey, Department of National Defence

The year 2010 has seen a number of arduous searches for missing aircraft lasting many days. This is why we often reiterate the message below. While it may seem repetitive to some, the single life this article may save in the future is certainly worth the half-page it is printed on. —Ed.

Most of us know that since February 1, 2009, emergency transmissions on 121.5 MHz are no longer monitored by the Cospas-Sarsat System. This means that if an aircraft has an accident and a 121.5 MHz emergency locator transmitter (ELT) is activated, search and rescue (SAR) agencies will not be alerted via the satellite network and, as a result, no resources will be launched. With any luck, a high-flyer may pick up the signal and report it; however, this is unreliable and lengthy delays in SAR response may result. A 406 MHz radio beacon signal is very likely to be detected quickly, relayed to the Canadian Mission Control Centre (CMCC) and acted on immediately.

SAR experts agree that 406 MHz beacons are superior to 121.5 MHz models. They affirm that their effectiveness is further enhanced if the 406 MHz ELTs are properly registered with the Canadian Beacon Registry (CBR). When a CMCC operator opens a case on an ELT, the first thing he or she does is verify if the beacon is registered. If it is, the information from the registration form is invaluable to the case. The operator can quickly determine if it is a false alarm or a developing situation. Time is of the essence and an unregistered 406 MHz ELT will delay the investigation phase.

Roughly 18 000 beacons are currently registered; however, this represents only about 55 to 60 percent of the 406 MHz ELTs in use in Canada. Many owners are misinformed with respect to their aircraft registration. They believe if the aircraft is registered with Transport Canada, then so is their 406 MHz beacon. This is not the case. The only way to know if your beacon is registered is to contact the CBR. If you went through the effort of purchasing a 406 MHz ELT, it makes sense to register it. It is also important to re-register your ELT every year and to verify your 15-digit HEX code to ensure it matches your registration. An ELT registered under the wrong HEX code is tantamount to not having it registered at all.

Properly registered beacons improve response time thereby saving lives! In order to register your 406 MHz beacon, please visit, or call 1-877-406-SOS1 (7671), or e-mail It is simple and takes only ten minutes.

Looking for AIP Canada (ICAO) Supplements
and Aeronautical Information Circulars (AIC)?

As a reminder to all pilots and operators, AIP Canada (ICAO) supplements
and AICs are found online on the NAV CANADA Web site (
Pilots and operators are strongly encouraged to stay up to date with these
documents by visiting the NAV CANADA Web site, and following
the link to “Aeronautical Information Products.”

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