NTSB IAD01LA028 - On January 19, 2001, a commercial pilot learned a hard lesson about changes in barometric pressure when the PA-34 he was flying sustained substantial damage after impacting the ground during and ILS approach to Bluefield, WV (BLF). Fortunately, the pilot was not hurt during the accident.
The pilot received a preflight briefing that indicated IFR conditions prevailed for the entire area. He was also given a NOTAM, which reported that the Runway End Identifier Lights were out at BLF. Bluefield ASOS reported 100-foot ceilings and an altimeter setting of 29.78. An FBO employee reported that the visibility was ¼ mile with "a lot of fog", which extended to the ground.
The pilot attempted three ILS approaches. Upon reaching the decision height on the first two, the pilot indicated that all he saw was clouds. During the second missed approach, the pilot was considering diverting to Beckley, WV, when the FBO employee contacted him on the radio. The employee indicated that the runway lights were on the low setting, and asked the pilot if he would like the intensity increased. The pilot agreed, and began the third approach.
Upon reaching the decision height on the third approach, the pilot indicated he might have been looking for the lights, and not monitoring his altitude. He then felt trees "slapping" the airplane. Several seconds later, the aircraft impacted the ground.
During the investigation, it was noted that 29.97 was set into the Kollsman window on the aircraft's altimeter. The pilot obtained altimeter settings from both Indianapolis Center and the Bluefield ASOS, but could not remember what the setting was. About 1,200 feet from the wreckage tops of trees approximately 60-70 feet tall were found sheared off.
The NTSB determined the cause of this accident to be the pilot's failure to enter the correct altimeter setting, which resulted in flight below the decision height.
According to the AIM, Section 7-2-3 (b):
Once in flight, it is very important to frequently obtain current altimeter settings en route. If you do not reset 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. An inch error in the altimeter setting equals 1,000 feet of altitude. To quote an old saying: "GOING FROM A HIGH TO A LOW, LOOK OUT BELOW."
In this instance, the altimeter was off .19 inches, or about 190 feet. The decision height for the approach was 300 AGL. The pilot was flying 190 feet LOWER than his altimeter indicated, so when he reached the decision height he was approximately 100 feet AGL. Pilots operating IFR below 18,000 feet must ensure the proper altimeter setting is set in the Kollsman window.
This accident report as well as others can be found in ASF's Online Database.
More information can be found in ASI's Safety Advisor "Single Pilot IFR".