C172 flight instructor and student reported while conducting a night training flight in the traffic pattern; the flight instruments began indicating erroneously; giving unreliable airspeed and altitude information. While troubleshooting the situation; the flight crew continued safely to a landing.

Narrative

I was conducting a night training flight with a student and we had just completed a stop and go at an airport near our home base. The airport had an operating control tower.The student performed a normal landing; came to a full stop; and we commenced our takeoff roll. Within 15-20 seconds of becoming airborne; I noticed the VSI was indicating near zero climb and that the altimeter was not increasing as I would expect. I asked the student to check his pitch attitude and quickly glanced at the airspeed indicator and noticed that it; too; was slowly decaying; reading approx 65 knots and falling. I immediately took the controls at this point. The aircraft was still in a nose up climbing attitude so I leveled off to arrest the speed decrease and avoid a stall; but unfortunately the speed was not increasing as one would expect. My first reaction was to double check that the flaps were retracted; and I visually confirmed that they were indeed retracted (I had already visually confirmed that they were up before the takeoff roll but checked again). My second reaction was that we were having a partial loss of power; so my singular goal was to maintain level flight and maintain airspeed and assess landing options. It should be noted that there were minimal visual references outside given it was night; so judging our actual height and speed was challenging. Given the uncertainty of the problem we were dealing with; I advised tower and told him I was returning to land; and he cleared me to land on any runway. I looked at the tachometer and oil pressure gauges which were indicating normal range and stable. I also realized that the aircraft engine sounded like it was producing normal power; and flight controls were behaving normally. I cross-checked my flight instruments again and noticed that airspeed was now in the 30 knot range on the digital speed tape while continuing to decrease; and registering no reading on the analog airspeed indicator (effectively zero). It was at this point that I mentally shifted the diagnosis from power loss issue to a faulty instruments issue. I increased pitch attitude slightly to re-establish a climb and began a right turn to head back towards the airport (crosswind leg). It should be noted that everything described above; from the time I took the controls until I advised tower and started turning back; all happened within the span of 8-10 seconds; so things happened extremely fast. At this point I completely disregarded the altimeter; VSI; and airspeed indicator readings (both the digital and standby instruments) as it was clear there was some pitot-static failure and their readings were completely unreliable. While the outside air temp was approx 20 deg C; I turned on the pitot heat as a precaution to see if that would rectify anything; and it did not. By this point we were on the downwind leg and leveled off at what I estimated to be approximately pattern altitude. My iPad with GPS had gone to 'sleep'; so I turned it on to see what it was reading as my altitude and groundspeed; and saw that my altitude was reading aprox 1;200 MSL (consistent with pattern altitude) and airspeed in the normal 90 knot range. I asked Tower to confirm my altitude and to my surprise he was indicating 600' (this further confirmed my suspicion of a static port/line issue which may have been giving faulty reading via the transponder). For the remainder of the short flight; I had my student call out groundspeed readings from the GPS every few seconds as I descended from pattern altitude. Tower also made a call to me while on base; showing me descending thru 700' and 70kts groundspeed. I made a normal and otherwise uneventful landing; taxied off the runway and received clearance to parking. The tower confirmed with me that the issue was just instruments; and I confirmed that indeed my altimeter and airspeed indicators were inop. The tower had also dispatched emergency equipment; so an ARFF truck followed us back to parking and collected some basic information about the nature of the situation and left. My student and I debriefed extensively following the flight on what things went well; and what things we could have done differently. We both acknowledged that the fact that it was a dark night with minimal visual cues on takeoff; making reconciling what we were seeing on the instruments with what we saw outside more challenging. We also could have potentially troubleshooted more; but acknowledged that being so close to the ground the priority was to get away from obstructions quickly and return back for landing; as we had our initial doubts pertaining to engine power (which of course were later ruled out).Following this experience; I intend to impress upon my students the importance of feeling flight characteristics at various speeds with the airspeed indicator covered; and being able to make a landing with no airspeed indications.