Federal Investigators Examine Altimeter Discrepancies in D.C. Plane Crash Hearings

Altimeter Discrepancies in Helicopter Crash

• ✈️ Federal investigators are examining discrepancies in altimeter readings following a collision between an Army helicopter and a regional jet at Ronald Reagan National Airport.
• ⚠️ The helicopter was reportedly flying up to 100 feet higher than the allowed altitude limit, according to its barometric altimeter.
• 📈 Investigators noted that barometric altimeters showed readings 80 to 130 feet lower than the helicopter's determined altitude over the Potomac River.

Pilot Reliance on Altitude Instruments

• 🛩️ A U.S. Army helicopter test pilot stated that an 80-foot difference between altitude instruments would not be cause for alarm.
• 🧭 Pilots often rely more heavily on one instrument over the other based on the specific flight conditions and altitude.

Factors Contributing to Altitude Errors

• 🔧 Potential errors contributing to discrepancies include the Kolsman window setting, instrument error, and position error.
• 🌡️ Non-standard temperature can also affect readings, potentially causing altimeters to read lower.
• 📊 The combination of instrument error, position error, and temperature correction can lead to significant discrepancies, though these factors do not always align in the same direction.

Inherent Limitations of Barometric Altimeters

• ⚙️ Tests on similar helicopters showed comparable discrepancies, leading to the conclusion that these are inherent limitations of barometric altimeters.
• ⚠️ Aircrews should have an expectation of a plus or minus 100-foot margin of error with barometric altimeter systems.