The Problem The Flight Management System (FMS) failed to properly process the Present Position Location (PPL) as the F-22 Raptors crossed over the International Date Line. As a result, systems dependent upon accurate PPL failed…
Key Concepts & Algorithms Key Concepts • Two important concepts, related to Longitude (LON) areLocal time (LT) and Universal Time (UT) • Longitudedetermines the hour of the day--not the date. • TheInternational Date Line (IDL)determines the date. As we cross the IDL the date advances one day (going west) or goes back one day (going east). • Universal Time (UT) or GMT is the Standard used for Aviation. • The Earth is divided into (24) Time Zones. Each Time Zone (TZ) is 15° wide. Algorithms TZ = LON / 15 ° LT = UT +/- TZ LT = UT +/- (LON / 15 °)
Case in point… Given: UT = 2400z Sunday midnight TZ = LON / 15° LT = UT + TZ LT = UT +/- (LON / 15°) Happy Day Scenarios: 157°W TZ=157°W / 15° = 10W LT = 2400z - 1000z = 1400z Sunday 2:00pm 179°W TZ=179°W / 15° = 12W LT = 2400z - 1200z = 1200z Sunday noon 179°E 179°E / 15° = 12E LT = 2400z + 1200z = 1200z Monday noon Exception: 181°WTZ=181°W / 15° = 12W LT = 2400Z - 1200z = 1200z Sunday noon IDL 179 ° E 1200z Monday Noon 179 ° W 1200z Sunday Noon 157 ° W 1400z Sunday 2:00pm 181 ° W 1200z Sunday Noon
Recommendations… • Do ensure your requirements are Testable • Real Requirement: “The system shall process present position location accurately.” • Do include qualified Standards and Principles in your Requirements Specification • Avionics and Navigation Principles • Key Concepts & Algorithms • Avionics Standards • DO-178C Software Considerations in Airborne Systems and Equipment Certification • Avionics Regulatory and Advisory Agencies • Federal Aviation Administration (FAA)
Problem • The F-22 Raptor computer software failed when the aircraft crossed the International Date Line. IDL
What went wrong? • The Flight Management System failed to properly process the location data as the F-22 flew from Hickam AFB (157° W) and crossed over the International Date Line (both 180° W and 180° E).
International Date Line 1956 2007 1985 1884 1522 1937
Inadequate requirements? • The system shall calculate the aircraft’s present position latitude and longitude in degrees, minutes, and seconds. • The system shall display the aircraft’s present position latitude and longitude in degrees, minutes, and seconds.
Poorly developed code? • Rapidly changing technology from analogue to digital flight controls • Led to increasing software development • Software • Ada was mandated by government • Difficult to find/hire Ada developers