B737 - ACAS II Potential Interference between Cargo Door Modification and Upper Directional Antenna

Aircraft : B737-200
Defect : ACAS II Potential Interference between Cargo Door Modification and Upper Directional Antenna

Spurious ACAS II signals had been reported after aircraft undergone freighter modification. ACAS II system was installed at SZB just prior to freight modification. Main Cargo Door (MCD) hinge fwd end protrudes by about 1 inch at the fwd-most end.

ACAS II system was tested using T-49C Test Set for bearing testing and air mode (2500 feet) simulated using Pitot Static Test Set. Various situations/scenarios were checked to verify most probably cause of these spurious signals together with radio LAE.

Scenario 1 - ACAS II Upper Directional Antenna at the existing location (BS 385), carried out outside hangar.

1) A few seconds after switching 'ON' the ACAS II system (T-49C is still 'OFF'); IVSI displayed:
- Nuisance intruder (diamond shape with no identification and altitude) appeared outside the RA ring (about 45 degree) for a short while. After 1 second, it appeared at different angle (about 225 degree) and moved around the ring and disappeared.
- Sometimes this nuisance intruder (diamond shape with no identification and altitude) appeared more than one place (2 or 3) at the same time at various angles.

2) Using T-49C for bearing testing transmitted at angles of 0, 225, 270 and 315o below were displayed on the IVSI:
- Intruder with identification and altitude (as setting on T-49C) coming in from the correct angle and altitude as per setting. The intruder with the diamond shape (solid, white) changed to circle (Amber, solid) and aural warning (Traffic alert) was audible once intruder entered the 2 nm range ring. This is correct indication.

3) However, at angles of 45, 90, and 135 o, we found the following discrepancy on the IVSI:
- Intruder with identification and altitude (as setting on T-49C) as per (2) above; and at the same time, nuisance intruder with identification and altitude (as setting on T-49C) coming in from incorrect angle (180 degree from the transmitted angle) and disappeared once it enter the 2 nm range ring. This seems to be a reflected signal probably caused by the MCD door hinge. However the correct indication was not affected by this spurious signal.

4) Once in a while we saw the nuisance intruder (diamond shape with no identification and altitude) appeared outside the ring and disappeared after 1 second and re-appeared again for short durations at different angle (at various angles).

5) At the same time an actual aircraft was flying above our acft and we saw the correct traffic shown on IVSI (intruder with identification and altitude) coming from LEFT of the acft (at angle of 225 to 360 degree) with clean signal (no reflection or erratic). There was NO aural warning or traffic alert since aircraft is flying above our acft on ground.

Summary of Scenario 1
Based on the findings, the system is functional except the reflection image of intruder (nuisance) when an aircraft approaches at angles of 45, 90, and 135o. The nuisance intruder (TA diamond) disappears once it goes into the RA circle. The correct intruder is still shown on IVSI and provides a correct visual warning: blank diamond shape (white) changes to solid (white) and then once inside the 2 nm range ring, diamond shape changes to circle (Solid, Amber). Aural warning (TA and RA) is audible once intruder enters the 2 nm range ring. Correct indication maintains its true path. Therefore system is considered operative except for the spurious signals that typically last few seconds.

Scenario 2 - ACAS II Upper Directional Antenna at the existing location (BS 385) and the height is temporarily adjusted upwards by installing the 1 cm thick spacer. (Note: new bonding less than 2.5 milliohms as we use a wire jumper to bond to acft frame). Carried out on 31st Jan 2007 outside the hangar.
Findings as shown on the IVSI:

6) Same as Scenario 1 findings except this time we notice that there is NO nuisance intruder (diamond shape with no identification and altitude) on the IVSI per described in Scenario 1 para (1).

Summary of Scenario 2
Situation is only improved slightly with the antenna adjusted upwards.

Scenario 3 - ACAS II Upper Directional Antenna is moved fwd temporarily 2 feet from the existing location BS 385) and at the same stinger. (Note: new bonding less than 2.5 milliohms as we use a wire jumper to bond to acft frame). Carried out outside hangar.

7) Powered up with no signal from tester, same per described in Scenario 1 para (1).

8) Transmitted signal using T-49C at angles of 0, 225, 270 and 315o.
- Finding same as described in scenario 1 para (2). This is correct indication.

9) Transmitted signal using T-49C at angles of 45, 90, and 135o.
- Finding same as described in scenario 1 para (3). Correct indication with additional transient ghost indications.

10) Transmitted signal using T-49C directed to the cargo door hinge at angle of 90 degree (see figure)



IVSI displayed:
- Nuisance intruder with identification and altitude (as setting on T-49C) coming in at about 210 o i.e. from behind the acft and disappeared once it entered the RA ring.
- Nuisance intruder moved away from the acft at the same angle (210o).
- We believed this happened due to the reflection created by the MCD hinge.

11) During test we saw an intruder with identification and altitude (one aircraft was flying above) coming from the LEFT (at angle of 225 to 360o) with clean signal (no reflection or erratic)
At the same time, there was another aircraft flying above coming from the RIGHT (at angle of 30 to 135o). Traffic shown on IVSI was same per acft movement. After 1 second another ghost intruder with the same identification and altitude appeared at about 180o from the initial intruder at the same speed and disappeared once it enter the RA ring. We believed it was a reflection created by the MCD hinge.

Summary of Scenario 3
Moving the antenna forward did not solve completely the transient indication problem. Furthermore the fwd location cannot be used for antenna installation due to the 9G barrier attachments.

Scenario 4 – MCD hinge covered using a metal plate (5’X 3’). T-49C transmitting to the Upper ACAS II antenna. Carried out outside hangar.

12) Powered up with no signal from tester, same per described in Scenario 1 para (1).

13) T-49C transmits at 0º (about 5 meter) from Upper ACAS II Antenna
- Intruder (signal from T-49C) with correct direction and altitude was seen on IVSI.
- At the same time, acft taking off/landing from runway was also seen on IVSI.
- No nuisance image occurred.

14) T-49C transmits at 90º (about 5 meter) from Upper ACAS II Antenna
- Intruder (signal from T-49C) with correct direction and altitude was seen on IVSI. The correct intruder with the diamond shape (solid, white) changed to circle (Amber, solid) and aural warning (TA & RA) was audible once intruder entered the 2 mile range ring. This is correct indication.
- Nuisance intruder (Solid diamond shape) appeared at 315º + 1200 ft and disappeared after 1 second.
- At the same time there was a solid diamond at +1200 to +1300 ft and duplicated it self for a few seconds and disappeared.

15) T-49C transmits at 45º, 135º, 225º, 270º and 315º (about 5 meter) from Upper ACAS II Antenna
- T-49C ‘ON’. Intruder with correct direction and altitude was seen on IVSI
- No nuisance image occurred.

Summary of Scenario 4
Covering the MCD hinge with the metal plate removed the problem of duplicate image at opposite direction (at angles of 45 o, 90 o, and 135o). The correct intruder was shown on IVSI and provided a correct visual and aural warning. However, ghost (intruder with no ident i.e. direction and altitude) persisted when signal was at 90º; and the ghost indications were different to Scenario 1.



Summary/Discussion

1) Based on the findings above the system is considered operative except for the spurious signals that typically last few seconds. The system provides correct intruder identification and path, visual and aural warning. Blank diamond shape (white) changes to solid (white) and then once inside the 2 nm range ring, diamond shape changes to circle (Solid, Amber) and aural warning (TA and RA) are triggered.

2) All scenarios except 4 showed the reflected (nuisance) image of intruder once with simulated acft approaching at angles of 45, 90, and 135 degrees. The ghost intruder (TA diamond) disappeared once it entered 2 nm range ring.

3) Covering the MCD hinge with a metal plate (scenario 4) removed the problem of spurious duplicate image at opposite direction (at angles of 45, 90, and 135 degrees).

4) Moving the antenna forward did not solve the transient indication problem.

5) Nuisance intruder (no ident i.e. direction and altitude) appeared at all scenarios; but less when the MCD hinge was covered with metal plate.

6) The protruding MCD hinge has created reflected signals which caused transient indications of intruders.

7) There was no reply from other B737-200 freighter operators if they faced the same problem.

8) The most promising solution is covering the MCD hinge with some fairing; preferably metallic. However it would not eliminate completely spurious signals since simulated acft at 90 o caused nuisance indications.

9) Cause of transient indications during system start up is not identified; but the short duration does not cause a problem.

Conclusion

This problem of spurious and transient ACAS II indications is most probably caused by the protruding MCD hinge introduced by the freighter modification. However the ACAS II system is operative and providing correct warnings. Most promising solution is to install a metallic fairing over the MCD hinge and this will be pursued with AEI. However it would not eliminate all spurious signals. Educating the flight crew should prevent faulting the ACAS II system due to the spurious/transient indications described.