Navigation svstems using a GPS (ATA 34)

This Airworthiness directive applies to all IFR certified aircraft fitted with a navigation system including a GPS sensor listed in Appendix 1.

This Airworthiness Directive applies to GPS multisensor or standalone systems certified (see Flight Manual) as:

- supplemental IFR navigation systems in accordance with AC 20-138 or AC 20-130A.

- BRNAV systems certified in accordance with AMJ 20X2 or JAA TGL 2.

- primary remote area/ oceanic navigation systems certified in accordance with FAA order N8110-60.

During night from August 21 to 22, 1999, the GPS system week number will transition from 1023 to 1024, and the GPS EPOCH will change. Several GPS systems will erroneously consider week 1024 as week 0 and will consequently use the corresponding EPOCH 0, dated January 6,1980.

This week number roll over ( WNRO) may affect the navigation function, either by inducing a loss of position computation, or an erroneous position.

A test allows to check proper functioning of the equipment after WNRO. The use of the equipments not listed as WNRO immune is prohibited by this AD after August 18, 1999, until the test is performed satisfactorily. August 18, 1999 date has been retain because GPS receiver may anticipate almanach change until 70 hours before GPS End of Week.

3.1. The use of GPS systems is prohibited on and after August 18, 1999, until the test described in paragraph 3 is successfully passed.

3.2. Before August 1 st ,1999, install a placard near the navigation system control panel, stating

The reactivation of the GPS and the removal of the placard may not be performed until the test described in paragraph 3 is satisfactorily passed.

3.3. Test of proper functioning after WNRO

Switch on the navigation system and check the following :

* check that the GPS date is correct.

*check that the GPS time is correct ( if this cannot be displayed, check that the GPS position is correct, i.e check that the difference between the GPS position and the aircraft position does not exceed 0,5 NM).

*check that the GPS uses the satellites indicated by the new almanach, by comparing the satellites used by the GPS with those indicated by a GPS prediction software for the test date and time.

If not possible, check that the GPS position is correct (see above).

* check that there is no alarm or messages.