Narrative:

There were 3 expressions of ATC concern over maintaining minimum assigned altitudes during this flight. 2 expressions were on V190 en route from aeg, double eagle, albuquerque, to falcon field, ffz, phoenix. The other was during descent and approach from V190 to ffz. The manipulator of the controls was a private pilot in simulated instrument conditions under a hood. I was PIC. Apparently, the encoding transponder was indicating about 250-300 ft below the airplane altimeter. The airplane altimeter checked out within 30-40 ft of ground elevation during preflight checks. On 2 separate occasions, the plane was about 50 ft low per the aircraft altimeter but ATC said the reading was 300 ft low. I had flown the airplane 2 different times IFR in the weeks before this event and had not received any indication the transponder was reading low. The transponder and altimeter are within inspection periods. Approaching phoenix, the winds aloft were strong from the west and a mountain wave or some other phenomenon caused the airplane to lose about 100 FPM altitude for a short time. The airplane would not maintain assigned 12000 ft altitude even in a full power climb, so ATC temporarily cleared us to 11000 ft. The airplane was able to maintain that. When we were near phoenix we were stepped down again to 10000 ft. I communicated with ATC during the period of discussion over altitude. On handoff to phx approach, I told them that we were unfamiliar with phoenix and I appreciated any extra help. Phx acknowledged, gave headings and cleared us from 9000 ft to 6500 ft. We immediately started a descent. Very soon thereafter, we were cleared to 4500 ft on a new heading. To try to comply with these rapid descent instructions, we began a 1500 FPM descent. ATC gave us a direction and distance to look for ffz. We didn't see it. There was 3381 ft ridge between us and the airport. ATC turned us left during the descent to avoid the ridge and headed us more toward williams gateway (iwa). At this time I told the pilot to remove the hood and use visual flight rules. Shortly after we turned, ATC asked us for our altitude. I looked at the altimeter and it read over 4400 ft. I told them that. ATC said they had observed us as low as 4100 ft. I told them about the previous apparent 300 ft discrepancy. At the time of the less than 100 ft descent below minimums, the terrain under the aircraft and within several thousand ft laterally was less than 3000 ft, so terrain clearance was at least 1400 ft and as much as 2400 ft. Excellent VMC conditions existed and there was never the slightest danger of flight into terrain. Several factors contributed to the problem with minimums. One problem en route was that the aircraft was operating near its effective ceiling. I had asked several experienced pilots what altitude the airplane could operate at. Poh service ceiling is 16700 ft. I was told that 14000 ft was probably more realistic. In fact, 12000 ft was sometimes possible and sometimes marginal, but I didn't know that until I tried to fly that altitude. The airplane weighed about 200 pounds less than allowable gross weight and was in good operating condition. Another problem was that wbound, 12000 ft was the lowest altitude to file IFR. When I needed lower, ATC cleared me to 11000 ft for some periods of time. In the future, I will ask ATC for lower when the MEA or MOCA permits even if it does not comply with directional rules. ATC will not grant the request if it cannot assure separation. I had never flown this route before, but the aircraft owner had over 10 times. He recommended that I file IFR. In the future, using this airplane I would strongly consider using different routing at a lower altitude and flying VFR with flight following. I believe that would let me fly lower and stay better within the operating capability of the aircraft while still using the benefits of ATC. IFR rtes require very high altitudes for MEA and MOCA. During the flight along V190, I received several reports of light to moderate turbulence over mountains. I only encountered light turbulence. However, strong westerly winds created downdrafts that while not turbulent did cause problems in maintaining altitude. In the future, I will strongly consider changing my routing or delaying the flight if I hear turbulence reports when I'm operating near the limits of the aircraft. For a short period just before phoenix, we were in IMC. I was able to hold altitude during IMC. I had heard reports of IMC earlier and that influenced my decision to continue according to the flight plan. However, in the future I'd consider canceling IFR while clear of IMC and either waiting or changing rtes to continue VFR at a lower altitude that the plane could maintain readily. The possible flight below minimums on approach to phoenix was primarily a result of a change in focus. The pilot had been under the hood and doing a good job of holding altitude when able. Upon changing to visual rules, starting a turn, looking for the airport, worrying about class B airspace and a first-time approach to a new airport, the pilot's attention wandered just long enough outside the cockpit to push the 4500 ft minimum by not leveling off in time. I saw that the pilot caught and corrected the problem before ATC asked us about altitude. If we were below minimums, it was for seconds. In the future, it is apparent that it is vital to maintain full attention on the aircraft even though there are distrs. In addition, it would have been better to have canceled IFR in the excellent WX conditions and continued VFR because it would have involved fewer inputs and distrs for the crew to manage. In the future, I will be alert and caution fellow pilots about the dangers of changing focus when flight conditions change. In later talking to experienced pilots, it became obvious to me that I was overly concerned with complying with ATC clrncs as I came into a strange airport. I should have told ATC the descent rate was more than I felt comfortable with and asked for different or clarifying instructions. I did tell ATC that we were descending at 1500 FPM to try to comply. The incidents arose from several causes. One was a possible discrepancy in the transponder altimeter. Another was operating near the limits of aircraft performance. Another was operating under unfamiliar conditions in unfamiliar surroundings. Another was uncertainty in expectations. As a result of this series of events, I will be even more conservative about aircraft performance and will be more outspoken in expressing my reservations or questions about ATC instructions. In addition, I will be sure to consider and change flight plans earlier if my expectations don't seem borne out by reality.

Google
 

Original NASA ASRS Text

Title: C172 INSTRUCTOR AND STUDENT HAD A MODE C READOUT ABOUT 300 FT LOW. THEY ALSO WERE TRYING TO OPERATE THE ACFT AT 12000 FT.

Narrative: THERE WERE 3 EXPRESSIONS OF ATC CONCERN OVER MAINTAINING MINIMUM ASSIGNED ALTS DURING THIS FLT. 2 EXPRESSIONS WERE ON V190 ENRTE FROM AEG, DOUBLE EAGLE, ALBUQUERQUE, TO FALCON FIELD, FFZ, PHOENIX. THE OTHER WAS DURING DSCNT AND APCH FROM V190 TO FFZ. THE MANIPULATOR OF THE CTLS WAS A PVT PLT IN SIMULATED INST CONDITIONS UNDER A HOOD. I WAS PIC. APPARENTLY, THE ENCODING XPONDER WAS INDICATING ABOUT 250-300 FT BELOW THE AIRPLANE ALTIMETER. THE AIRPLANE ALTIMETER CHKED OUT WITHIN 30-40 FT OF GND ELEVATION DURING PREFLT CHKS. ON 2 SEPARATE OCCASIONS, THE PLANE WAS ABOUT 50 FT LOW PER THE ACFT ALTIMETER BUT ATC SAID THE READING WAS 300 FT LOW. I HAD FLOWN THE AIRPLANE 2 DIFFERENT TIMES IFR IN THE WKS BEFORE THIS EVENT AND HAD NOT RECEIVED ANY INDICATION THE XPONDER WAS READING LOW. THE XPONDER AND ALTIMETER ARE WITHIN INSPECTION PERIODS. APCHING PHOENIX, THE WINDS ALOFT WERE STRONG FROM THE W AND A MOUNTAIN WAVE OR SOME OTHER PHENOMENON CAUSED THE AIRPLANE TO LOSE ABOUT 100 FPM ALT FOR A SHORT TIME. THE AIRPLANE WOULD NOT MAINTAIN ASSIGNED 12000 FT ALT EVEN IN A FULL PWR CLB, SO ATC TEMPORARILY CLRED US TO 11000 FT. THE AIRPLANE WAS ABLE TO MAINTAIN THAT. WHEN WE WERE NEAR PHOENIX WE WERE STEPPED DOWN AGAIN TO 10000 FT. I COMMUNICATED WITH ATC DURING THE PERIOD OF DISCUSSION OVER ALT. ON HDOF TO PHX APCH, I TOLD THEM THAT WE WERE UNFAMILIAR WITH PHOENIX AND I APPRECIATED ANY EXTRA HELP. PHX ACKNOWLEDGED, GAVE HDGS AND CLRED US FROM 9000 FT TO 6500 FT. WE IMMEDIATELY STARTED A DSCNT. VERY SOON THEREAFTER, WE WERE CLRED TO 4500 FT ON A NEW HDG. TO TRY TO COMPLY WITH THESE RAPID DSCNT INSTRUCTIONS, WE BEGAN A 1500 FPM DSCNT. ATC GAVE US A DIRECTION AND DISTANCE TO LOOK FOR FFZ. WE DIDN'T SEE IT. THERE WAS 3381 FT RIDGE BTWN US AND THE ARPT. ATC TURNED US L DURING THE DSCNT TO AVOID THE RIDGE AND HEADED US MORE TOWARD WILLIAMS GATEWAY (IWA). AT THIS TIME I TOLD THE PLT TO REMOVE THE HOOD AND USE VISUAL FLT RULES. SHORTLY AFTER WE TURNED, ATC ASKED US FOR OUR ALT. I LOOKED AT THE ALTIMETER AND IT READ OVER 4400 FT. I TOLD THEM THAT. ATC SAID THEY HAD OBSERVED US AS LOW AS 4100 FT. I TOLD THEM ABOUT THE PREVIOUS APPARENT 300 FT DISCREPANCY. AT THE TIME OF THE LESS THAN 100 FT DSCNT BELOW MINIMUMS, THE TERRAIN UNDER THE ACFT AND WITHIN SEVERAL THOUSAND FT LATERALLY WAS LESS THAN 3000 FT, SO TERRAIN CLRNC WAS AT LEAST 1400 FT AND AS MUCH AS 2400 FT. EXCELLENT VMC CONDITIONS EXISTED AND THERE WAS NEVER THE SLIGHTEST DANGER OF FLT INTO TERRAIN. SEVERAL FACTORS CONTRIBUTED TO THE PROB WITH MINIMUMS. ONE PROB ENRTE WAS THAT THE ACFT WAS OPERATING NEAR ITS EFFECTIVE CEILING. I HAD ASKED SEVERAL EXPERIENCED PLTS WHAT ALT THE AIRPLANE COULD OPERATE AT. POH SVC CEILING IS 16700 FT. I WAS TOLD THAT 14000 FT WAS PROBABLY MORE REALISTIC. IN FACT, 12000 FT WAS SOMETIMES POSSIBLE AND SOMETIMES MARGINAL, BUT I DIDN'T KNOW THAT UNTIL I TRIED TO FLY THAT ALT. THE AIRPLANE WEIGHED ABOUT 200 LBS LESS THAN ALLOWABLE GROSS WT AND WAS IN GOOD OPERATING CONDITION. ANOTHER PROB WAS THAT WBOUND, 12000 FT WAS THE LOWEST ALT TO FILE IFR. WHEN I NEEDED LOWER, ATC CLRED ME TO 11000 FT FOR SOME PERIODS OF TIME. IN THE FUTURE, I WILL ASK ATC FOR LOWER WHEN THE MEA OR MOCA PERMITS EVEN IF IT DOES NOT COMPLY WITH DIRECTIONAL RULES. ATC WILL NOT GRANT THE REQUEST IF IT CANNOT ASSURE SEPARATION. I HAD NEVER FLOWN THIS RTE BEFORE, BUT THE ACFT OWNER HAD OVER 10 TIMES. HE RECOMMENDED THAT I FILE IFR. IN THE FUTURE, USING THIS AIRPLANE I WOULD STRONGLY CONSIDER USING DIFFERENT ROUTING AT A LOWER ALT AND FLYING VFR WITH FLT FOLLOWING. I BELIEVE THAT WOULD LET ME FLY LOWER AND STAY BETTER WITHIN THE OPERATING CAPABILITY OF THE ACFT WHILE STILL USING THE BENEFITS OF ATC. IFR RTES REQUIRE VERY HIGH ALTS FOR MEA AND MOCA. DURING THE FLT ALONG V190, I RECEIVED SEVERAL RPTS OF LIGHT TO MODERATE TURB OVER MOUNTAINS. I ONLY ENCOUNTERED LIGHT TURB. HOWEVER, STRONG WESTERLY WINDS CREATED DOWNDRAFTS THAT WHILE NOT TURBULENT DID CAUSE PROBS IN MAINTAINING ALT. IN THE FUTURE, I WILL STRONGLY CONSIDER CHANGING MY ROUTING OR DELAYING THE FLT IF I HEAR TURB RPTS WHEN I'M OPERATING NEAR THE LIMITS OF THE ACFT. FOR A SHORT PERIOD JUST BEFORE PHOENIX, WE WERE IN IMC. I WAS ABLE TO HOLD ALT DURING IMC. I HAD HEARD RPTS OF IMC EARLIER AND THAT INFLUENCED MY DECISION TO CONTINUE ACCORDING TO THE FLT PLAN. HOWEVER, IN THE FUTURE I'D CONSIDER CANCELING IFR WHILE CLR OF IMC AND EITHER WAITING OR CHANGING RTES TO CONTINUE VFR AT A LOWER ALT THAT THE PLANE COULD MAINTAIN READILY. THE POSSIBLE FLT BELOW MINIMUMS ON APCH TO PHOENIX WAS PRIMARILY A RESULT OF A CHANGE IN FOCUS. THE PLT HAD BEEN UNDER THE HOOD AND DOING A GOOD JOB OF HOLDING ALT WHEN ABLE. UPON CHANGING TO VISUAL RULES, STARTING A TURN, LOOKING FOR THE ARPT, WORRYING ABOUT CLASS B AIRSPACE AND A FIRST-TIME APCH TO A NEW ARPT, THE PLT'S ATTN WANDERED JUST LONG ENOUGH OUTSIDE THE COCKPIT TO PUSH THE 4500 FT MINIMUM BY NOT LEVELING OFF IN TIME. I SAW THAT THE PLT CAUGHT AND CORRECTED THE PROB BEFORE ATC ASKED US ABOUT ALT. IF WE WERE BELOW MINIMUMS, IT WAS FOR SECONDS. IN THE FUTURE, IT IS APPARENT THAT IT IS VITAL TO MAINTAIN FULL ATTN ON THE ACFT EVEN THOUGH THERE ARE DISTRS. IN ADDITION, IT WOULD HAVE BEEN BETTER TO HAVE CANCELED IFR IN THE EXCELLENT WX CONDITIONS AND CONTINUED VFR BECAUSE IT WOULD HAVE INVOLVED FEWER INPUTS AND DISTRS FOR THE CREW TO MANAGE. IN THE FUTURE, I WILL BE ALERT AND CAUTION FELLOW PLTS ABOUT THE DANGERS OF CHANGING FOCUS WHEN FLT CONDITIONS CHANGE. IN LATER TALKING TO EXPERIENCED PLTS, IT BECAME OBVIOUS TO ME THAT I WAS OVERLY CONCERNED WITH COMPLYING WITH ATC CLRNCS AS I CAME INTO A STRANGE ARPT. I SHOULD HAVE TOLD ATC THE DSCNT RATE WAS MORE THAN I FELT COMFORTABLE WITH AND ASKED FOR DIFFERENT OR CLARIFYING INSTRUCTIONS. I DID TELL ATC THAT WE WERE DSNDING AT 1500 FPM TO TRY TO COMPLY. THE INCIDENTS AROSE FROM SEVERAL CAUSES. ONE WAS A POSSIBLE DISCREPANCY IN THE XPONDER ALTIMETER. ANOTHER WAS OPERATING NEAR THE LIMITS OF ACFT PERFORMANCE. ANOTHER WAS OPERATING UNDER UNFAMILIAR CONDITIONS IN UNFAMILIAR SURROUNDINGS. ANOTHER WAS UNCERTAINTY IN EXPECTATIONS. AS A RESULT OF THIS SERIES OF EVENTS, I WILL BE EVEN MORE CONSERVATIVE ABOUT ACFT PERFORMANCE AND WILL BE MORE OUTSPOKEN IN EXPRESSING MY RESERVATIONS OR QUESTIONS ABOUT ATC INSTRUCTIONS. IN ADDITION, I WILL BE SURE TO CONSIDER AND CHANGE FLT PLANS EARLIER IF MY EXPECTATIONS DON'T SEEM BORNE OUT BY REALITY.

Data retrieved from NASA's ASRS site as of July 2007 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.