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Acute Fatigue in Pilots

Two scales frequently used in fatigue studies are the Samn-Perelli 7-point fatigue scale(S-PS) (Samn & Perelli, 1982) and Karolinska sleepiness scale (KSS) (Akerstedt & Gillberg, 1990). The S-PS asks pilots to rate their current level of arousal on a 7-point scale while the KSS asks pilots to rate their level of sleepiness on a 9-point scale. These scales, then, tap into the individual’s current status and their acute fatigue, and a higher score indicates a worse condition. Both instruments are well established in aviation fatigue studies and have been validated against objective measures.

S-PS scores across two sectors

FIGURE 4.1 S-PS scores across two sectors.

I looked at the results of 125 pilots operating a 2-sector day from their home base (Figure 4.1). The working day was fairly routine in terms of start and end times, and no crew flew during the night. I found a moderate negative association between the hours of sleep in the previous 24 (-0.4128 p < 0.05) and 48 (-0.4877 p < 0.05) hours and S-PS scores. The less sleep pilots had before a duty, the worse was their perception of their state of arousal (higher scores). There was a weak positive association between the hours awake before duty and the S-PS for all pilot groups. The longer pilots were awake before their duty, the more likely they were to feel more fatigue. There was a statistically significant (p < 0.045) difference in S-PS scores between the sectors, with higher scores recorded in the second sector. Half of the pilots also completed the fatigue severity scale (FSS), and there was a moderate association between their S-PS and FSS scores (0.42886, p 0.00329). These data suggest that sleep duration, hours of wakefulness and duration of duty affect pilots’ acute fatigue state, but the FSS scores indicate that pilots’ chronic fatigue state will also be a moderating factor. An S-PS score of 4 is considered the point at which fatigue is starting to become significant. Of the total sample, 16% scored 4 on the SPS while 31.8% scored 1 on the scale, indicating that they felt fully alert. This suggests that under routine daytime conditions a significant number of pilots are operating in a marginal state.

Powell et al. (2008) looked at S-PS scores in a large sample of pilots flying two- crew regional operations. The scores were taken at the top of descent. The highest levels of fatigue were observed in the window of circadian low (WOCL) (0200-0600) and were also associated with the length of the duty day. Fatigue was 0.56 points higher on the S-PS at the end of two sectors compared with a single-sector duty. Aeschbach et al. (2017) found a relationship between the length of the duty, sectors flown and fatigue as measured with the S-PS. Fatigue increased by 0.4 points for every hour awake and 0.3 points for every sector flown. Sallinen et al. (2017) used the KSS to study pilot fatigue and, similarly, found that duty periods that covered the whole local night-time period (i.e. 00:00-06:00 at the domicile) were consistently associated with reduced subjective alertness.

Types of flying and SPS scores

FIGURE 4.2 Types of flying and SPS scores.

Figure 4.2 shows data for a larger sample of regional daytime flights and also includes data for crews flying two-sector night flights. The night flying crews provided SPS data at sign-on (before flight) and after landing for each sector. There was a statistically significant increase in S-PS scores between pre-flight and landing for the first sector but no significant increase across the second sector. These data reinforce the earlier findings that acute fatigue is cumulative and that night-time operations increase the exposure. The apparent plateauing of fatigue in the night flying crews was possibly the effect of a small sample size.

Acute fatigue in pilots, then, is a function of hours of sleep and time awake before a duty, the length of the duty period, the number of sectors flown and the time of day. Furthermore, acute fatigue is cumulative.

Acclimatisation and Night Flying

Long-haul flying and transiting through multiple time zones result in circadian arrhythmia, or ‘jet- lag’. In a study of cargo pilots operating in Asia and North America, the crew were asked to complete the S-PS and also to report whether they felt that they were acclimatised to the local time zone. As a very rough rule of the thumb, it takes 1 day per time zone crossed to become acclimatised. In reality, the process is hugely variable, so I relied on each individual to judge their personal state. The first comparison made was of the difference between a sample of acclimatised passenger aircraft pilots flying daytime operations and the cargo crews who typically flew through the night. Table 4.2 shows the distribution of S-PS scores in the two groups. For comparison, data for a regional LCC are included (Yen et al., 2009).

The data suggest that, in a group of acclimatised pilots, 17.6% of pilots operating during daytime and 31.25% of pilots working night schedules experienced excessive acute fatigue (S-PS > 4). The comparison with the Low Cost Carrier (LCC) pilots is interesting and might reflect the impact of high rotation, multiple sector days. Different types of flying, then, have different fatiguing characteristics.


Distribution of S-PS Scores (%)




Regional LCC





















When I looked at the difference between the acclimatised and non-acclimatised cargo pilots, I found that there was a moderate positive association between the hours awake before the duty and the S-PS score in non-acclimatised pilots (0.49, p <0.05). The longer they had been awake, worse was their perceived state of arousal. However, there was no difference between the groups in terms of the average actual hours of wakefulness when compared to their daytime passenger fleet colleagues (passenger 3.34 hours and cargo 3.23 hours). It seems, then, that fatigue builds faster in non-acclimatised pilots. Furthermore, the difference between all-cargo pilots (acclimatised but operating night flights; not acclimatised operating local time night flights while on home base daytime body clock) and the acclimatised pilots flying daytime regional flights in terms of their SPS scores was significant. Pilots who are not acclimatised to the local time zone are starting the day in a worse fatigue state than the acclimatised crew, and night operations seem to be equivalent to flying non-acclimatised.

These results point to the complicated nature of sleep and fatigue. Pilots encounter acute fatigue (as measured by the SPS) routinely in the workplace. Acute fatigue is cumulative across shifts and is related to prior sleep and wakefulness. It is also related to the nature of work (night flying; high rotation) and also the extent to which the pilots are acclimatised to the local time. Having looked at acute fatigue as experienced by pilots during working duties, I now want to move on to chronic fatigue.

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