Air Safety- Altimeter Errors

Few pilots would disagree that the three point altimeter is a particularly difficult instrument to interpret during stressful and/or time constrained situations. The reason that we have this difficulty however, stems from a number of issues relating to the way that we conceptualise changes in altitude. Firstly, we don’t generally think of increasing altitude in terms of a circular motion such as we would the compass heading. We generally conceptualise increasing altitude in terms of a vertical linear motion. Indeed, this is the preferred method of displaying altitude in aircraft equipped with EFIS technology (refer Figure Two).

The second reason for the difficulty in interpreting three point altimeters involves the nature of the pointers themselves. Normally, we think of altitude as a single unit such as seven thousand, five hundred feet (7500ft) or nine thousand, one hundred feet (9100ft). The three point altimeter however dissects this information into three distinct components with the smallest needle indicating tens of thousands of feet, and the remaining needles indicating thousands and hundreds of feet respectively. Somehow, pilots are expected to integrate this information and this, unfortunately, takes time and is likely to result in errors.

Undoubtedly, the distinction between the three needles reflects the somewhat ad hoc development of the three point altimeter from the first years of aviation. Quite simply, it was inconceivable, during the early years of aviation, for aircraft to exceed 1000ft above ground. Altimeters therefore, only required a single needle designed to indicate 100s of feet above the reference point. As aircraft became more advanced however, and exceeded both the 1000 and 10,000ft barriers, the need arose to augment existing altimeters. The simplest and most cost effective solution in this case was to include additional needles.

The final reason for the difficulty associated with the three point altimeter involves the dial and the display of numerals. Our familiarity with the analog clock faces of watches would suggest that the numeral six on the altimeter display should be placed in a position inline with the vertical. Instead, the three point altimeter is based upon the decimal system of displays in which the numeral five, rather than six, occupies this position. Whilst this might appear to represent a minor difficulty in terms of a 100 foot error, consider the implications in terms of a 1000 or 10,000 foot error.

Whilst it would be unreasonable at this stage for all general aviation aircraft to incorporate the EFIS type display of altitude, a practical alternative may exist in the form of the combination analog/digital display (refer Figure Three). This instrument is designed to combine the advantages associated with the analog type display with those associated with the digital type displays.

The University of Newcastle recently conducted a series of trials designed to evaluate the differences between two different types of altimeters; the three point altimeter and the combination digital/analog altimeter. These represent the two most common types of altitude displays utilised in general aviation aircraft. The main goal was to determine whether any differences existed between the ability of inexperienced pilots to accurately report the altitude indicated by the various displays. A secondary goal however, was to determine whether inexperienced pilots could learn to overcome the problems associated with three point altimeters.

The two types of displays were presented using a computer and pilots were asked to assume that they were travelling en route through controlled airspace when their altitude was requested by air traffic control. In the first series of forty trials, pilots were given two seconds in which to view each altimeter before a response was required. This was reduced to one second for the second series of forty trials.

The results of the research for the two second display indicated that pilots misread the three point altimeter by an average of 3500ft. In contrast, pilots misread the combined analog/digital display by an average of 500ft. As expected therefore, inexperienced pilots made more severe errors using the analog display than using the combined analog/digital display. In terms of the time taken to respond, it took pilots an average 5.5 seconds to respond to the three point altimeter, whereas it took pilots only 4.3 seconds on average to respond to the combined digital/analog display. Once again the results highlight the relative difficulty which inexperienced pilots face when interpreting three point altimeters.

The results of the research for the one second display indicated that pilots misread the three point altimeter by an average of 500ft but misread the combined analog/digital display by an average of only 100ft. Consistent with the results for the two second display, pilots during the one second display took an average of six seconds to respond to the analog display and an average of 3.5 seconds to the digital display.

The results of the analysis indicated two important points about the responses to analog and digital altimeters. Firstly, as might be expected, inexperienced pilots made fewer errors with the combination analog/digital altimeter than with the three point, analog altimeter during both the one second and two second display. In addition, it took comparatively less time for pilots to respond to the combination altimeter. This is clear evidence of the difficulties which inexperienced pilots face with the accurate and timely interpretation of three point altimeters.

Secondly, we might have expected that during the one second display, pilots would have made more errors than during the two second display. An examination of the results for the two series however, indicates that this was far from the case. For the analog displays, pilots made relatively fewer errors during the one second displays but they took longer to respond. Since the two second series of altimeters preceded the one second series, the results suggest that pilots recognised the problems they were having with the three point altimeters and compensated for this by taking their time to respond. In this way, they made fewer errors overall.

The results therefore revealed an unexpected benefit associated with the computer programme: that an exposure to a series of computer based trials can increase an inexperienced pilot’s awareness of the inherent difficulties associated with three point altimeters. More importantly, it can allow inexperienced pilots the opportunity to develop strategies which overcome the inherent problems. Short of changing all the three point altimeters to combination analog/digital displays, creating an awareness amongst inexperienced pilots is the next best alternative.

Many of the world’s accidents involving three point altimeters have been due to the pilot misreading the altimeter by 10,000 feet. This is probably due to the relative size of the smallest needle coupled with the problems of stress and/or fatigue. In 1958 for example, a BEA Viscount collided with terrain having been cleared to descend from 18,500 feet to 8000 feet. According to the accident investigation, the aircraft had crashed soon after the pilots had reported “passing 11,000 feet” and it appeared likely that the crew had misread by 10,000 feet, the shortest needle on the aircraft’s three point altimeter.

In this study, we tried to increase the relative stress on pilots by reducing the amount of time they had to examine the altimeter. Whilst this would have been expected to increase the number of 10,000ft errors made by pilots, the results indicated that the average number of 10,000ft errors was in fact, halved. Therefore, by developing strategies to cope with instruments such as the three point altimeter, pilots can also reduce the effects of stress on their performance during flight.

In human factors training, one of our biggest problems is making the information “real” to pilots. It is comparatively easy to lecture to pilots about the inherent difficulties associated with different types of altimeters than to provide them with practical experience. In our situation, there was no aircraft and nor were there the problems such as noise and high workload, which are normally associated with operations in an aircraft. Pilots were provided with experience in a controlled environment in which they could concentrate on a single task: reading the altimeter. Clearly, the next step in this process of human factors education is to reinforce the same principles during pilots’ flight training. Research suggests that it is not sufficient to simply use a computer programme and expect pilots to develop skills which are directly transferable to the cockpit.

Initially, the results should be discussed with the student and an emphasis placed on the connection between the computer based training and operations in the cockpit. Students should be encouraged to discuss their results and why or why not they may have made errors. It is only through this continuous process of performance assessment that inexperienced pilots themselves will understand and recognise their own limitations.

The principle described here of providing pilots with practical human factors knowledge and skills is one which should underpin all future human factors training programmes. This does not mean however, that vast hours of costly flight training time needs to be dedicated specifically to human factors training. Rather, computer based instruction in human factors can provide the basis for future inflight training in which human factors principles are integrated into existing programmes. It is only through the provision of as much practical experience as possible that human factors training will have the desired impact and reduce the proportion of aircraft crashes for which human error is considered the probable cause.

Note: This research would not have been possible without the assistance of students in Human Factors at the University of Newcastle’s Department of Aviation.