Narrative:

I was providing instrument instruction but my student wasn't doing a good job tracking the localizer. We wanted to discontinue but the controller gave us a heading to re-intercept instead. Since the situation was not improving I decided to take the controls from the student but got confused on how far were we in the localizer and started my descent as if I were already past the FAF. Just when I noticed how far in reality I was and about 1;300 feet AGL the controller called us to climb immediately back to 1;800 feet (the prescribed altitude for that segment of the approach) which we immediately complied with. The approach was finished uneventfully after that and landed safely at our destination.I lost situational awareness momentarily due to the fact that so many things were going on at the same time plus the fact that I started doubting my instruments because the GPS installed in the airplane was giving a different information of our position in relation to the localizer. I know that I should rely on my instruments at all times but the altimeter and vsi had been acting weird jumping up and down a couple of hundred feet for very brief and sporadic periods of time.the solution I'm going to implement to prevent this from ever happening again is to not start a descent until I verbally manifest my position on the approach and challenging myself or whoever I'm flying with that we can do so and not just assume it. Also I will discontinue an approach if I don't feel confident on it regardless of suggested headings from the controller to re-intercept.

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Original NASA ASRS Text

Title: Flight instructor reported a premature descent after assuming control of the aircraft when the instrument student pilot was unable to track the localizer.

Narrative: I was providing instrument instruction but my student wasn't doing a good job tracking the localizer. We wanted to discontinue but the controller gave us a heading to re-intercept instead. Since the situation was not improving I decided to take the controls from the student but got confused on how far were we in the localizer and started my descent as if I were already past the FAF. Just when I noticed how far in reality I was and about 1;300 feet AGL the controller called us to climb immediately back to 1;800 feet (the prescribed altitude for that segment of the approach) which we immediately complied with. The approach was finished uneventfully after that and landed safely at our destination.I lost situational awareness momentarily due to the fact that so many things were going on at the same time plus the fact that I started doubting my instruments because the GPS installed in the airplane was giving a different information of our position in relation to the localizer. I know that I should rely on my instruments at all times but the altimeter and VSI had been acting weird jumping up and down a couple of hundred feet for very brief and sporadic periods of time.The solution I'm going to implement to prevent this from ever happening again is to not start a descent until I verbally manifest my position on the approach and challenging myself or whoever I'm flying with that we can do so and not just assume it. Also I will discontinue an approach if I don't feel confident on it regardless of suggested headings from the controller to re-intercept.

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.