Toronto Runway 05 – ITX05 CRJ False Capture Approach At
To measure for False courses on a localizer the Flight Inspection aircraft is flown in a fashion that it arcs through the extended runway centerline. The arc begins at a horizontal offset of 35 degrees from the runway centerline and extends to 35 degrees on the opposite side of the centerline. While in flight, sensors record the Difference in Depths of Modulation (DDM) as it would be seen by standard aviation receivers. As per ICAO, Annex 10 Volume #1, the international standard for off course clearance measurements is; Therefore once the DDM has reached 0.180 (175ua) due an aircraft being horizontally offset from the runway centerline, the DDM must remain at that level or increase until the angular offset is 10° from the runway centerline. From then on to 35°, the DDM could drop to 0.155DDM (150ua) (see note #2) Although the tolerance for the minimum DDM value between 10° and 35° is set in paragraph 220.127.116.11.4 to 0.155DDM, it is recommended a minimum of 0.180DDM us used instead. This recommended value is what is used in Canada International Standard
DDM refers to the difference in modulation depths between the 90Hz and 150Hz tones that modulate the carrier frequency. Modulation values are expressed in percentages and for localizers are set to a nominal value for 20% each. Without going into the complexity of how the modulation values are altered in the air, the goal is to produce a signal that will appear to have a varying modulation value such that it creates a 0 difference (DDM=0.0000) on the exact runway centerline. From the centerline outwards, the radiated signal varies in a fashion that will have the 90Hz component increasing in its percentage of modulation (as referenced to the 150Hz component) on the left side of the runway (while on approach). The right side of the runway will be the opposite where the 150Hz component is greater in modulation values than the 90Hz component. To calculate the DDM value simply take the modulation value for the 90Hz component and subtract the 150Hz component. Ex: Right of center, the 90Hz = 12% (or 0.12) and 150Hz =28% (or 0.28) DDM = 0.12-0.28 = -0.16 DDM values are further converted into microamps (ua) which are used to deflect analog style indicators. For localizers, a full scale deflection equates to a DDM value of 0.155 and requires 150ua of current to move the needle to the full scale deflection. Therefore to convert the DDM value to microamps (ua), multiply the DDM by the conversion factor of (150/0.155) Ex: from above, -0.16 * (150/0.155) = -155ua Explanation of DDM
Two frequency systems refer to ground equipment radiating two distinct carrier frequencies. The signals are referred to as REFERENCE and CLEARANCE. Through a complex antenna combining network these two signals are radiated in a fashion that will have a predominant REFERENCE (REF) signal on the centerline through the proportional guidance sector (where the aircraft indications are not a their full scale limits). The CLEARANCE (CLRNC) signal is most predominate outside of the proportional guidance sector and is captured by the airborne receiver to provide a hard fly left of hard fly right indication dependent upon which side of the runway the aircraft is on. Extended runway centerline LOC +4° +6° +10° or +35°, depending on distance Aircraft flying in this region will be Processing the CLEARANCE signal Aircraft flying in this region will be Processing the CLEARANCE signal -4° -6° -10° or -35°, depending on distance Aircraft flying in this region will be processing the REFERENCE signal Explanation of Two Frequency Systems
Front Course Off Course Clearance (Wide) TOLERANCE Linear increase in CP from course line to 175ua (accepted) Maintain not less than 175ua to ±10° (passed) Maintain a minimum of 150ua between ±10° and ±35° (passed) Area where CP must be linear Must be below -150ua Must remain below -175ua Must be above 150ua Must remain above 175ua Results of airborne data collection December 28 th after numerous reports were received. The Clearance arc clearly demonstrated that the radiated signal was within tolerance DDM values recorded by airborne receiver converted to microamps (ua) CP recorded by FI receiver (ua) in this area is approximately 220ua (0.227DDM)
Aircraft positioning for LOC intercept Aircraft begins to roll so that they position themselves on a heading that will take them on a course to be 30° of the runway centerline, this will initiate a 30° roll left The Aircraft roll continues in the ambigous CRSE/CLRNC zone and creates a DIP in the crosspointer current (DDM) that will be low enough to instruct the Flight Guidance system to capture the signal. Because the aircraft is not on course, the captured DDM is not providing a decreasing DDM therefore signals the flight guidance system that it is not in the correct location and aborts the intercept.
To simulate this NVC101 (CRJ) was dispatched January 17, 2011 in an attempt to better understand the situation and collect more data. The procedure would be to position our aircraft under the same conditions that commercial CRJs were exposed to when the false captures were reported and repeat these manoeuvres until our autopilot captured and begins to intercept prior to the actual centerline. There were a number of runs performed. The first two runs did not produce the results, however the third run clearly demonstrated the fault with the Autopilot capturing and tracking what it thought was the course when in fact it was not. The 4 th run did produce similar results however the autopilot did not capture. It is believed that this was because the DDM value recorded did not drop below the minimum level required for the autopilot to capture.
This run did have the autopilot capture prematurely. The false course dip was recorded by the FI receiver and clearly shows that the receiver is responding to the CLEARANCE signal. The assumption here is that the CLEARANCE dip is directly a function of aircraft roll in this 4 to 6 degree horizontal offset area. The above example shows that the aircraft was in a near 30 degree roll for approximately 15 seconds. CP recorded by FI receiver (ua), dip is 193.6ua (0.200DDM) Aircraft is rolling for approximately 15 secs GPS Vert = 1.93° GPS Distance = 14.4nm
CP recorded by FI receiver (ua), dip is 196.3ua (0.203DDM) This run did not have the autopilot capture prematurely. The false course dip was recorded by the FI receiver and clearly shows that the receiver is responding to the CLEARANCE signal, however was not low enough (as compared to the previous plot) to trigger the autopilot to capture. The assumption here is that the aircraft roll was not as long in duration as the previous run and did not affect the signal to the same magnitude. GPS Vert = 1.91° GPS Distance = 14.4nm
While the GND based system was operating well within ICAO guidelines, we modified the radiated signal in manner that would overcome the effect of the aircraft roll on the CLEARANCE DDM. This procedure involved increasing what is known as the CLEARANCE SBO power. While this is a minor adjustment, the effect could be major if the increased power creates high reflections in the off course sectors. To test for this, the equipment was flown in a number of conditions to ensure the increased power did not create any true false courses. In should be noted, that due to the limited amount of testing done, the absolute effect of the aircraft roll on the DDM has not been established and the ground based equipment adjustments may not solve this condition in all cases. GND Equipment Adjustment
Original Clearance Arc in Width Normal TOLERANCE Linear increase in CP from course line to 175ua (accepted) Maintain not less than 175ua to ±10° (passed) Maintain a minimum of 150ua between ±10° and ±35° (passed) Area where CP must be linear Must be below -150ua Must remain below -175ua Must be above 150ua Must remain above 175ua Results of airborne data collection December 28 th after numerous reports were received. The Clearance arc clearly demonstrated that the radiated signal was within tolerance DDM values recorded by airborne receiver converted to microamps (ua) CP recorded by FI receiver (ua) in this area is approximately 220ua (0.227DDM)
GND equipment operating at new WIDE limit CP recorded by FI receiver (ua) in this area is approximately 285ua (0.295DDM) The CP recorded in this run is 65ua higher than the data collected in December in the same area. This increase in CP should be large enough to overcome the dip created by rolling aircraft. Must be below -150ua Must remain below -175ua Must be above 150ua Must remain above 175ua GPS Vert = 1.98° GPS Distance = 14.3nm
With GND equipment operating in Normal With a limited aircraft roll ( 5° left ) the dip in the cross pointer is not observed. GPS Vert = 1.97° GPS Distance = 14.3nm
With GND equipment operating in Normal The following 4 runs were flown to try and recreate the false capture by the cockpit receiver. The autopilot capture could not be recreated, however in looking at the data it was clear that the signal is still subject to interference with roll. Another question arises when the horizontal angle is compared for the natural dip. Not all four runs show this dip to be at the same horizontal offset angle. With an aircraft roll >24° left, the dip in the cross pointer is still repeatable.
With GND equipment operating in Normal With an aircraft roll >24° left, the dip does not drop below 200ua with the GND equipment operating in its normal condition. GPS Vert = 2.18° GPS Distance = 12.4nm
With GND equipment operating in Normal With an aircraft roll >24° left, the dip does not drop below 200ua with the GND equipment operating in its normal condition. GPS Vert = 2.12° GPS Distance = 12.8nm
With GND equipment operating in Normal With an aircraft roll >24° left, the dip does not drop below 200ua with the GND equipment operating in its normal condition. GPS Vert = 2.31° GPS Distance = 11.8nm
To overcome the affects of aircraft roll, the GND equipment was adjusted so that the dip in the cross-pointer (DDM) would remain above 200ua. With the adjustment made, numerous runs were flown while rolling the aircraft through the critical zone. While each run did show the dip, the autopilot never did capture the Localizer signal. The GND equipment was also configured in maximum and minimum operating conditions to ensure the false captures could not be recreated with allowable operating conditions. These runs also demonstrated that although the dip was still observed, the level did not reach levels low enough to allow the autopilot to capture. With the conditions on the ITX05 system mitigated, the findings did reveal questions as to why the dip occurs and what conditions contribute to this condition. To test for this, tests will be performed to better understand the mechanics involved in this condition. Summary of work done in Toronto ITX05
What exactly is the problem? Rate of change??
Conclusions: o Really none at this point – work is ongoing, - tested other facility (same type) did not see same effect -small possibility of ground equipment being cause basically restricted to polarization or, site frequency - most probably issue is aircraft based - Avionics based (capture trigger point or rate of change) - physical aircraft (hull reflections or antenna null patterns) (eliminated shadowing effect)