Track angle error (TAE) displays and their effect on pilot performance during instrument approaches
Abstract:
According to the FAA's Technical Standard Order (TSO) C129a, Global Positioning System (GPS) based area navigation (RNAV) devices used for non-precision approaches under Instrument Flight Rules (IFR) must be able to display Track Angle Error (TAE). The TSO requires that at least numeric TAE should be available but suggests that analog TAE may be more desirable. TAE has potential to be beneficial to pilots because it provides Cross-Track Error (XTE) derivative information for use in the tracking manual control loop. However, in traditional General Aviation (GA) aircraft, this supplementary TAE information can usually only be displayed on the front panel of the RNAV unit itself, a location which forces the pilot to widen his instrument scan. A previous study (Oman, Huntley and Rasmussen, 1995) showed that analog TAE information improves tracking performance during simulated GPS instrument approaches. The purpose of the present
study was to compare tracking performance and workload using four different display formats: one showing TAE in numeric form and three which presented TAE in a variety of analog formats. Twelve pilots each flew 16 non-precision approaches in a modified Frasca 242 flight simulator. During the initial portions of the approach, when the sensitivity of the XTE display was low, use of one of the analog TAE displays (a "track vector" format) produced as much as a 20% decrease in root mean square (RMS) XTE and up to a factor of two improvement in the width of XTE envelopes (estimated 95% limits). The analog TAE formats increased both Bedford workload ratings (up to 18%) and aircraft RMS roll angle. After being artificially displaced 0.25nm off the final approach course, pilots using the "XTE predictor" analog format re-intercepted at a consistently shallower angle. Independently of which display they were using, the subgroup of pilots with better than average inner loop roll control performance achieved superior outer loop XTE tracking. When using any of the three analog TAE formats this pilot subgroup also improved their performance as they established themselves on final approach. Ranking the displays on both XTE performance and workload ratings the "track vector" format was best. It was followed by the numeric TAE format, the XTE predictor format and finally a dual pointer XTE/TAE format. This project was supported by the Department of Transportation Contract DTRS-57-92-C-0054 TTD#27B to the MIT Center for Transportation Studies.
