Narrative:

Completed all preflight duties; including initialization of FMS and verification that navigation mode was indicating GPS and Q value <10. Input flight plan per release/clearance and verified SID and routing. Completed passenger and baggage loading; received closeout; and proceeded with normal pushback; engine start; and departure from ramp. ATC assigned us runway 17 for departure. Completed normal taxi operations and before takeoff checklists; including verification of RNAV SID and that procedure heading was indicated on navigation page 1. Received take off clearance and started takeoff roll. Proceeded with normal liftoff and initial climb and subsequent engagement of LNAV mode on FMS. I was pilot flying; first officer was pilot not flying. Completed acceleration altitude procedures and initiated second stage climb segment. At about 1;900 ft MSL both magenta cdis suddenly went from a centered position to a left-of-course deviation. Cas CDI indicated approximately 1 dot left deviation; first officer's CDI indicated an approximately 1 1/2 dot left deviation. The FMS wind readout on cas pfd now displayed a direct right cross wind of 73 KTS; while the first officer's pfd displayed a direct right crosswind of 102 KTS. The FMS command bars then directed a turn to the right to re-intercept the course. As I initiated the turn to the right; I commented to the first officer about the FMS wind speed and direction indicated on the pfd's. I questioned whether that readout could be accurate considering our low altitude (1;900 ft MSL) above the ground. The FMS command bars rolled level on an approximate heading of 180 degrees for the intercept course. Departure control then called us and inquired if we were direct to the first waypoint. I quickly verified that we were still in LNAV mode; that both mfd's and navigation pages indicated direct to the waypoint; that my magenta CDI was centered (first officer's CDI was still offset approximately 1/2 dot left of course); and then had the first officer reply to ATC that yes; we were direct. ATC then stated that it looked like we were drifting west of course and directed us to fly a 130 degree heading. We placed the FMS in heading mode and turned to the assigned heading. ATC then further directed us to turn left direct another waypoint further out. We entered direct to an FMS fix into the FMS and engaged the LNAV mode again. FMS command bars turned to the left; mfd's and navigation pages indicated direct; so since everything looked correct; I engaged the autopilot. Departure control then had us contact center. This entire event took place within in the first five minutes after takeoff. After the first officer had checked in with center; we started discussing what could have happened on the departure. I commented again on the unusually high wind readout that was indicated on the pfd's; and concluded that the FMS had analyzed that wind speed and direction and was simply trying to re-intercept the course line. We then wondered about the overall accuracy of the FMS and decided to go to navigation page 2/2 on both mfd's for verification. The navigation mode now indicated DME/DME on both FMS's; with a Q=2 on my side and a Q=3 on the first officer's side. Since I knew they were supposed to be indicating GPS; I then went to data page 2/4 and saw the GPS 1 fail / GPS 2 fail messages indicated on both FMS's. I remarked to the first officer that both GPS's were not working; so the aircraft didn't know its exact position; and this is why the FMS had indicated inaccurate course directions after takeoff. The first officer then commented that just prior to rotation during the takeoff roll he had noticed the FMS message light blinking; but when he pressed the message key button; there was no message displayed. He stated that he thought the message light had come on in error. We then deduced that this must have been the point in time when both GPS's failed for unknown reasons causing the subsequent course inaccuracies. The first officer then questioned whether we should continue to our destination because the weather there was not forecast to be good; and we were going to have to shoot a GPS approach due to a runway closure. I decided to see if we could troubleshoot the problem and get the GPS's reinitialized in order to continue the flight. I told the first officer that if they wouldn't come back online; we would have to return to the departure airport. I then directed the first officer to turn his FMS completely off; wait about 10 seconds; and then turn it back on. His FMS reinitialized; and the GPS now indicated that it was working correctly and the navigation mode again displayed GPS. He then cross filled the flight plan to his GPS; after which I coupled the autopilot to his side. I then turned my FMS off; waited 10 seconds and then turned mine back on. After initialization; my GPS also indicated it was now working correctly again. I cross filled the flight plan back from FMS2 and re-coupled the autopilot to my side. Since the issue had been resolved; I felt comfortable about continuing the flight. I asked the first officer for his opinion and he concurred. This process had taken about 15 minutes to complete. Just as we finished fixing our problem; center called us and informed us that departure control had contacted them and wanted to give us a phone number to call after we landed to discuss the course deviation. After we arrive; I called the number but it went to voice mail. I left them my name and number and informed them that I would be on the ground for approximately 30 minutes. No one returned the call before we left; so after we arrived back the original departure airport I placed a second call to departure control; but again it went to voice mail so I left a second message. I had finished flying so I went home. About two hours later I received a call from tower operations manager; and we discussed the event. I explained to him that we had had a dual GPS failure on takeoff with no crew notification or alerts; and how that had subsequently caused the FMS to indicate inaccurate course directions which led to the deviation. He said he understood; but stated that since separation had been lost; that he would have to forward the information to the local FSDO. I gave him my information and thanked him for the call.

Related reports:

Google
 

Original NASA ASRS Text

Title: Both GPS's failed on takeoff causing an EMB145 track deviation. ATC detected the deviation before the crew identified the equipment malfunction.

Narrative: Completed all preflight duties; including initialization of FMS and verification that NAV mode was indicating GPS and Q value <10. Input flight plan per release/clearance and verified SID and routing. Completed passenger and baggage loading; received closeout; and proceeded with normal pushback; engine start; and departure from ramp. ATC assigned us Runway 17 for departure. Completed normal taxi operations and before takeoff checklists; including verification of RNAV SID and that procedure HDG was indicated on NAV page 1. Received take off clearance and started takeoff roll. Proceeded with normal liftoff and initial climb and subsequent engagement of LNAV mode on FMS. I was pilot flying; First Officer was pilot not flying. Completed acceleration altitude procedures and initiated second stage climb segment. At about 1;900 FT MSL both magenta CDIs suddenly went from a centered position to a left-of-course deviation. CAS CDI indicated approximately 1 dot left deviation; First Officer's CDI indicated an approximately 1 1/2 dot left deviation. The FMS wind readout on CAS PFD now displayed a direct right cross wind of 73 KTS; while the First Officer's PFD displayed a direct right crosswind of 102 KTS. The FMS command bars then directed a turn to the right to re-intercept the course. As I initiated the turn to the right; I commented to the First Officer about the FMS wind speed and direction indicated on the PFD's. I questioned whether that readout could be accurate considering our low altitude (1;900 FT MSL) above the ground. The FMS command bars rolled level on an approximate heading of 180 degrees for the intercept course. Departure Control then called us and inquired if we were direct to the first waypoint. I quickly verified that we were still in LNAV mode; that both MFD's and NAV pages indicated direct to the waypoint; that my magenta CDI was centered (First Officer's CDI was still offset approximately 1/2 dot left of course); and then had the First Officer reply to ATC that yes; we were direct. ATC then stated that it looked like we were drifting west of course and directed us to fly a 130 degree heading. We placed the FMS in heading mode and turned to the assigned heading. ATC then further directed us to turn left direct another waypoint further out. We entered direct to an FMS fix into the FMS and engaged the LNAV mode again. FMS command bars turned to the left; MFD's and NAV pages indicated direct; so since everything looked correct; I engaged the autopilot. Departure Control then had us contact Center. This entire event took place within in the first five minutes after takeoff. After the First Officer had checked in with Center; we started discussing what could have happened on the departure. I commented again on the unusually high wind readout that was indicated on the PFD's; and concluded that the FMS had analyzed that wind speed and direction and was simply trying to re-intercept the course line. We then wondered about the overall accuracy of the FMS and decided to go to NAV page 2/2 on both MFD's for verification. The NAV mode now indicated DME/DME on both FMS's; with a Q=2 on my side and a Q=3 on the First Officer's side. Since I knew they were supposed to be indicating GPS; I then went to data page 2/4 and saw the GPS 1 fail / GPS 2 fail messages indicated on both FMS's. I remarked to the First Officer that both GPS's were not working; so the aircraft didn't know its exact position; and this is why the FMS had indicated inaccurate course directions after takeoff. The First Officer then commented that just prior to rotation during the takeoff roll he had noticed the FMS MSG light blinking; but when he pressed the MSG key button; there was no message displayed. He stated that he thought the MSG light had come on in error. We then deduced that this must have been the point in time when both GPS's failed for unknown reasons causing the subsequent course inaccuracies. The First Officer then questioned whether we should continue to our destination because the weather there was not forecast to be good; and we were going to have to shoot a GPS approach due to a runway closure. I decided to see if we could troubleshoot the problem and get the GPS's reinitialized in order to continue the flight. I told the First Officer that if they wouldn't come back online; we would have to return to the departure airport. I then directed the First Officer to turn his FMS completely off; wait about 10 seconds; and then turn it back on. His FMS reinitialized; and the GPS now indicated that it was working correctly and the NAV mode again displayed GPS. He then cross filled the flight plan to his GPS; after which I coupled the autopilot to his side. I then turned my FMS off; waited 10 seconds and then turned mine back on. After initialization; my GPS also indicated it was now working correctly again. I cross filled the flight plan back from FMS2 and re-coupled the autopilot to my side. Since the issue had been resolved; I felt comfortable about continuing the flight. I asked the First Officer for his opinion and he concurred. This process had taken about 15 minutes to complete. Just as we finished fixing our problem; Center called us and informed us that Departure Control had contacted them and wanted to give us a phone number to call after we landed to discuss the course deviation. After we arrive; I called the number but it went to voice mail. I left them my name and number and informed them that I would be on the ground for approximately 30 minutes. No one returned the call before we left; so after we arrived back the original departure airport I placed a second call to Departure Control; but again it went to voice mail so I left a second MSG. I had finished flying so I went home. About two hours later I received a call from Tower Operations Manager; and we discussed the event. I explained to him that we had had a dual GPS failure on takeoff with no crew notification or alerts; and how that had subsequently caused the FMS to indicate inaccurate course directions which led to the deviation. He said he understood; but stated that since separation had been lost; that he would have to forward the information to the local FSDO. I gave him my information and thanked him for the call.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.