The three-pointer dial altimeter (with a shortest pointer indicating tens of kilofeet, a short pointer indicating kilofeet, and a long pointer indicating hectofeet), the most instantly-recognisable style of analog barometric altimeter, is also, apparently, the type most vulnerable to misreading, with a number of accidents either definitively attributed to, or suspected to have been caused by, pilots misreading three-pointer altimeters (frequently in the form of believing one’s altitude to be ten kilofeet higher than one it actually is).1
- A BOAC Bristol Britannia on a test flight crashed on 24 December 1958 while on approach to land; testimony from surviving crewmembers definitively identified a misread altimeter as the cause of the crash.
- United Airlines Flight 389 crashed into Lake Michigan on 16 August 1965 after the flightcrew failed to arrest its descent at the altitude to which they had been cleared; the cause of the continued descent into the water was never conclusively determined, but a misread altimeter is strongly suspected.
- Iberia Airlines Flight 62 descended below its clearance limit on 4 November 1967 and crashed into a hill in West Sussex; the cause of the crash is not known with certainty, but one of the possibilities is a misread altimeter.
Further, to quote the NTSB report on the crash of UA389:
As a part of this investigation a review of research on altimeter reading problems was conducted. There are some fifty studies relating to the various altimeter systems of presenting information to pilots which have been conducted since 1947. One of the most recent studies completed by the Naval Research Laboratory, Washington, D. C. 16/ was conducted to determine the relative effectiveness of various altimeter displays. These investigations, consisted [sic] of laboratory research at NRL and flight test [sic] at a nearby naval air facility, were in support of a Department of Defense program. Four types of altimeter presentations, the counter-pointer (CP), counter-drum-pointer (CDP), drum-pointer (DP) and three pointer (3P) were compared in a series of laboratory experiments measuring reading time and accuracy with both pilots and non-flying enlisted men. The same instruments were also compared by the pilots in flight, using the questionnaire technique to elicit pilot judgments. The results of the various phases of laboratory research showed that the three-pointer altimeter consistently ranked poorest. The pilot preference in the flight tests was overwhelmingly in favor of one of the other types of altimeter. In all objective measurements taken, the three-pointer altimeter ranked the poorest among the instruments tested. The finding that the three-pointer was poorest proved to be statistically reliable in every study in which tests of statistical significance were applied. In a preceding part of this study 17/ the mean exposure time and number of errors on each altimeter was recorded for the two test groups, one consisting of pilots, and the other of non-pilot enlisted men. In 1080 readings of the four altimeters by 18 pilots the mean exposure time in seconds was 0.80 for the CP, 0.84 for the CDP, 1.38 for the DP, and 2.28 for the 3P. The number of reading errors was 7 for the CP, 14 for the CDP, 26 for the DP, and 80 for the 3P. The same data collected during 420 reading trials using 7 non-pilot enlisted men resulted in mean exposure times of 0.85 for the CP, 0.86 for the CDP, 1.50 for the DP, and 2.93 for the 3P. The number of errors by this group were 5 for the CP, 2 for the CDP, 10 for the DP, and 52 for the 3P. The mean number of errors made on each altimeter for pilots was 0.39 CP, 0.78 CDP, 1.44 DP, and 4.44 3P, and for enlisted men 0.71 CP, 0.29 CDP, 1.43 DP, and 7.43 3P. [Pages 24-25 (paper report)/28-29 (PDF).]
This is quite surprising, given that the three-pointer altimeter display works on the same principle as an analog clock face, and even nowadays (with the proliferation of digital clock displays), the vast majority of people can rapidly and accurately read the time displayed on an analog clock; back in the 1960s (before the digital clock was a consumer item), practically every literate adult would have been able to do this. It shouldn’t be hard to translate those skills into reading altitude rather than time, so that, instead of (say) 3 hours (shortest pointer) 25 minutes (short pointer) 32 seconds (long pointer) past midnight, you have (say) twenty- (shortest pointer) -five thousand (short pointer) seven hundred (long pointer) feet above mean sea level.