A lot of technology is available to piston pilots these days. This includes certified glass-panel displays in new airplanes as well as gee-whiz, almost-real-time weather graphics displayed on the same laptop computer used to play MP3s over the ship’s audio system. Some of the portable products available to the GA aircraft owner and pilot these days are truly revolutionary and can literally mean being able to complete a marginal-weather trip safely and reliably instead of holing up somewhere and waiting for the bad stuff to pass.The additional capability these portable devices bring to the average GA cockpit is a definitely a good thing. But — you knew there would be a “but,” right? — they aren’t certified as a primary navigation source under IFR. Consequently, they should only be used in supporting the equipment already installed in your panel.While there are certainly legal and safe ways to, say, use a handheld GPS to navigate under IFR to a destination hundreds of miles away, it’s not a good idea to use portable equipment as the primary resource in more safety-critical applications, like instrument approaches. In addition to the accuracy and reliability of such equipment, its design and user interface can be less-than-ideal, leading to distractions. As a result, depending on portable, non-certified equipment as a primary source of navigation under IFR in anything other than an emergency is not only illegal but it’s not a good idea.This latter point was demonstrated on the afternoon of October 15, 2002, when a Cessna 172N was destroyed while attempting the GPS Runway 21 approach to the Mount Sterling-Montgomery County Airport (IOB) in Mount Sterling, Ky. The Private pilot and his passenger were fatally injured. The daytime flight departed the Greenwood Municipal Airport (HFY) in Greenwood, Ind., and was conducted under an IFR clearance.
Reported weather at IOB about 30 minutes before the crash included a broken ceiling at 500 feet AGL and another broken cloud layer at 1600 feet AGL, with visibility of three statute miles. Some 30 minutes after the accident, IOB’s reported weather included an overcast ceiling at 500 feet AGL with the same visibility. Given the reported ceiling, the 172 would be 46 feet below the clouds when at the straight-in MDA of 1460 feet after a perfectly flown approach. That’s very doable with the proper equipment and a reasonably competent pilot.In this case, the pilot held a Private certificate with an Instrument rating and had accumulated some 478 total hours of flight experience; he had 2.6 hours of experience in a Cessna 172. The pilot had logged a respectable 106 hours of instrument time, 52 hours of which were in actual conditions. The pilot owned a Piper PA-32-260, which was down for maintenance. For the accident flight, he had rented the 172, with a CFI performing a checkout flight with him the night before.According to the NTSB, the CFI “reported that the pilot had demonstrated multiple IFR tasks to standard, and described the pilot as ‘very competent.’ The instructor also stated that the pilot used a handheld personal computer with a GPS …” One reason the accident pilot used his handheld computer with GPS on the checkout flight is because the 172 was not equipped with an IFR-certified GPS receiver.As illustrated, the final approach course for Mount Sterling’s GPS Runway 21 approach includes FELPO; ISFUR, the final approach fix; an unnamed step-down fix 2.3 nm from the missed approach point; and RW21, the missed approach point. The minimum descent altitude (MDA) at FELPO is 3000 feet MSL. The MDA between FELPO and ISFUR is 2600 feet MSL, and between ISFUR and the unnamed step-down fix it is 1800 feet MSL. The MDA between the unnamed step-down fix and RW21, for the airspeed flown, was 1460 feet MSL.The NTSB’s review of radar data revealed that the airplane passed abeam FELPO about one nm to the right of the approach course at 2700 feet. The airplane then tracked toward the approach course between FELPO and ISFUR with its altitude fluctuating between 2500 and 2300 feet. When the airplane was almost abeam ISFUR, it was about 1/2-mile to the right of the approach course, at 2300 feet. At that point, ATC advised the pilot that radar contact was lost and the pilot acknowledged the radio call. There were no further communications from the pilot. Earlier, the controller advised the pilot that the Lexington, Ky., altimeter setting was 29.85, but the pilot did not acknowledge the transmission.Shortly thereafter, the Cessna flew into a 415-foot-high tower about 75 feet from the top. The tower rose to 1426 feet MSL just slightly left of the approach course centerline and is depicted on the approach’s plan view; just past it is the unnamed step-down fix. The outboard section of the left wing, outboard section of the left horizontal stabilizer with elevator attached, and another portion of the left horizontal stabilizer, were found beneath the tower.A handheld personal computer, with a yoke mount and an antenna integrating a GPS receiver, were found in the cockpit and just outside the main wreckage. Examination of the airplane’s instruments revealed the altimeter indicated 1070 feet, with an altimeter setting of 29.96 displayed in the Kollsman window. If the pilot flew the approach with that setting, the airplane would have been 110 feet lower than the altimeter indicated.
Although it appears the probable cause of this accident was a descent below the MDA (the NTSB had not published a probable cause finding in this accident as of Aviation Safety’s deadline), whether the pilot’s use of a non-approved GPS receiver was a factor may never be known. In its factual report on this accident, the NTSB noted that the handheld computer’s GPS software was accompanied by a warning stating, “The system is not tested or approved by the FAA or any governmental agency and should not be used as a primary flight instrument.”The NTSB also saw fit to quote the Aeronautical information Manual: “If you do not set your altimeter when flying from an area of high pressure into an area of low pressure, your aircraft will be closer to the surface than your altimeter indicates.”The mis-set altimeter is definitely a clue to deciphering this accident. Presuming an error-free system aboard the 172, the altimeter would have indicated 1460 feet — exactly the MDA past the unnamed step-down fix — when the airplane hit the tower at 1351 feet MSL. If the pilot had not descended too early, the aircraft would have been at 1690 feet MSL (i.e., 110 feet below the MDA of 1800 feet MSL) for that segment of the approach.Additionally, the pilot was routinely as much as 300 feet below the minimum altitudes for the various approach segments. Some, but not all, of that error can be blamed on the mis-set altimeter. Ultimately, though, the pilot descended well below the MDA after passing ISFUR and struck a clearly charted tower along the final approach path.Since there was no other receiver aboard, it’s clear the pilot used his portable GPS equipment as a primary source of navigation while executing the approach. What we don’t know is what the handheld computer showed the pilot during the approach: Did it display the unnamed step-down fix? Did the pilot have a current approach plate displaying the step-down fix? Was the aircraft’s displayed position further down the final approach path than it really was? In other words, did the pilot think he was past the unnamed step-down fix allowing descent to 1460 feet?What we do know is that this accident wouldn’t have happened if the approach had been flown at the correct MDA. We also know it might not have happened if the 172 had been equipped with an IFR-certified GPS receiver approved for approaches.Read the NTSB Probable Cause Report here.