Narrative:

We took off for a maintenance flight after performing not only scheduled inspections; but also installing an overhauled GPS and multi function display. We filed for FL270. We initialized our equipment as normal and following checklist procedures. Right after passing 10;000 feet we noticed a problem with pressurization; so we changed to final altitude of 16;000 feet; then we requested 14;000 feet. Right after we leveled off; we were notified that we were at 13;600 feet; even though both of our altimeters were reading 14;000 feet. They made us switch transponder from alt mode to normal mode (C to a mode). We checked our first intersection; a little off but within tolerance; but reaching ZZZ VOR the airplane past a few miles then started turning southeast bound with a left turn taking us off our filed flight plan for a few miles (8 to 10 NM). ATC immediately called our attention vectoring us to a heading of 270 degrees and after a few miles they wanted us direct to ZZZ1 VOR. It was then when we realized that our GPS system and multifunction display had our airplane located like 50 miles west of the airport. Our immediate action was to cancel IFR and request VFR flight following back to the airport. After landing; maintenance crew took action to troubleshoot; finding out that our GPS antenna was damaged and not working properly; we evidently were not receiving satellite signals. We called the center supervisor to explain what happened and apologize for the inconvenience we caused; I also provided all my contact information including my pilot's certificate number.

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Original NASA ASRS Text

Title: CE550 Captain experiences track and altitude deviation during a maintenance test flight after scheduled inspections and the installation of an overhauled GPS and Multi-Function Display unit. The GPS antenna was found damaged on post flight.

Narrative: We took off for a maintenance flight after performing not only scheduled inspections; but also installing an overhauled GPS and Multi function display. We filed for FL270. We initialized our equipment as normal and following checklist procedures. Right after passing 10;000 feet we noticed a problem with pressurization; so we changed to final altitude of 16;000 feet; then we requested 14;000 feet. Right after we leveled off; we were notified that we were at 13;600 feet; even though both of our altimeters were reading 14;000 feet. They made us switch transponder from alt mode to normal mode (C to A mode). We checked our first intersection; a little off but within tolerance; but reaching ZZZ VOR the airplane past a few miles then started turning southeast bound with a left turn taking us off our filed flight plan for a few miles (8 to 10 NM). ATC immediately called our attention vectoring us to a heading of 270 degrees and after a few miles they wanted us direct to ZZZ1 VOR. It was then when we realized that our GPS system and multifunction display had our airplane located like 50 miles west of the airport. Our immediate action was to cancel IFR and request VFR flight following back to the airport. After landing; maintenance crew took action to troubleshoot; finding out that our GPS antenna was damaged and not working properly; we evidently were not receiving satellite signals. We called the Center supervisor to explain what happened and apologize for the inconvenience we caused; I also provided all my contact information including my pilot's certificate number.

Data retrieved from NASA's ASRS site 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.