Narrative:

I was pilot in command on a VFR flight. I performed most of the flight under the hood since I was building up simulated instrument and cross country time towards my instrument rating. The safety pilot had a private asel certificate and was instrument rated and current. The flight was uneventful until we approached the huntsville area. I contacted huntsville approach before entering the class C airspace and checked the mdq AWOS-3 for the weather at that time; which was about 20 minutes before the actual landing. I recall that the weather at mdq was above VFR minimums at that time although the dew point/temperature spread was near zero. I asked huntsville approach for radar vectors to perform a practice ILS approach under the hood and land at mdq while remaining VFR. The approach controller vectored us onto the localizer and then approved us to change frequencies and squawk VFR for the remainder of the flight. I announced my intentions on CTAF and began the approach under the hood. I told the safety pilot that I would lift the hood just before reaching the decision altitude for that approach and then perform a normal landing. I activated the pilot controlled lighting for mdq and the safety pilot informed me that he couldn't see any airport lights or the rotating beacon. I activated the lights again and he indicated that he still could not locate any airport lighting. I assumed he simply couldn't pick out the airport lights among the other lights in the area. In fact; what had happened was that patchy fog had developed on the ground including right over mdq and the lights simply weren't visible until we got near the decision altitude. The fog was about 200 feet thick; above about 200-300 ft AGL; visibility was good and the weather remained well above VFR minimums. I continued the approach and pulled the hood up just before reaching decision altitude. The safety pilot and I realized at that point that we had in fact just entered the fog bank just above the ground. I briefly considered performing a missed approach; however; the runway end identifier lights and runway lights became visible and I elected to perform a landing which happened uneventfully despite the fog. I should have broken off the approach when it became apparent that the weather right at the ground was no longer VFR. At that point; I could have climbed where the weather remained VFR and then asked my safety pilot to obtain an IFR clearance (thus making him the pilot in command since he was rated and current) and then either he or I could have flown the approach again to mdq or diverted to hsv where the weather was better. I learned that fog can develop very rapidly on clear calm nights even if the weather above 200 ft AGL is VFR and that frequent weather checks are necessary when the dew point/temperature spread is very narrow.

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Original NASA ASRS Text

Title: A private pilot on a monitored IFR training flight flew into IMC conditions while making a practice ILS approach.

Narrative: I was pilot in command on a VFR flight. I performed most of the flight under the hood since I was building up simulated instrument and cross country time towards my instrument rating. The safety pilot had a private ASEL certificate and was instrument rated and current. The flight was uneventful until we approached the Huntsville area. I contacted Huntsville Approach before entering the Class C airspace and checked the MDQ AWOS-3 for the weather at that time; which was about 20 minutes before the actual landing. I recall that the weather at MDQ was above VFR minimums at that time although the dew point/temperature spread was near zero. I asked Huntsville Approach for radar vectors to perform a practice ILS approach under the hood and land at MDQ while remaining VFR. The Approach Controller vectored us onto the localizer and then approved us to change frequencies and squawk VFR for the remainder of the flight. I announced my intentions on CTAF and began the approach under the hood. I told the safety pilot that I would lift the hood just before reaching the decision altitude for that approach and then perform a normal landing. I activated the pilot controlled lighting for MDQ and the safety pilot informed me that he couldn't see any airport lights or the rotating beacon. I activated the lights again and he indicated that he still could not locate any airport lighting. I assumed he simply couldn't pick out the airport lights among the other lights in the area. In fact; what had happened was that patchy fog had developed on the ground including right over MDQ and the lights simply weren't visible until we got near the decision altitude. The fog was about 200 feet thick; above about 200-300 FT AGL; visibility was good and the weather remained well above VFR minimums. I continued the approach and pulled the hood up just before reaching decision altitude. The safety pilot and I realized at that point that we had in fact just entered the fog bank just above the ground. I briefly considered performing a missed approach; however; the runway end identifier lights and runway lights became visible and I elected to perform a landing which happened uneventfully despite the fog. I should have broken off the approach when it became apparent that the weather right at the ground was no longer VFR. At that point; I could have climbed where the weather remained VFR and then asked my safety pilot to obtain an IFR clearance (thus making him the pilot in command since he was rated and current) and then either he or I could have flown the approach again to MDQ or diverted to HSV where the weather was better. I learned that fog can develop very rapidly on clear calm nights even if the weather above 200 FT AGL is VFR and that frequent weather checks are necessary when the dew point/temperature spread is very narrow.

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.