From the Fatigue Risk Management Committee - 8/17/16
From the Fatigue Risk Management Committee - 8/17/16


Stay informed!
Sign up for Texcaster
Edit Email Info
Edit Contact Info

What follows below is the second article in a series of articles from the Fatigue Risk Management Committee intended to help shed some new light on fatigue-related items that you may not have thought of before. If you missed the first article and would like to read it, please click here.

John Cardaci
MEC Vice Chairman

Impact of Fatigue
I am continually amazed at the challenging job we do every day, under less than ideal conditions, even after 20 years of flying the line.

As a new hire, I did not realize how ignorant I was about the impact of fatigue and the importance of fatigue-mitigation strategies. Now as I approach retirement, I’d like to share some of the lessons I have learned and how they compare with the NASA sleep study conducted in 1987–89 using FedEx pilots.


1. Due to our circadian cycle, flying on the backside of the clock is the most dangerous time to operate, regardless of the amount of recent sleep. Error rates increase, and cognitive effectiveness decreases significantly during this time because of the circadian sleep impulse. The worst performance occurs in the Window of Circadian Low (WOCL), which makes it the most dangerous time in which to operate. Add sleep debt and sleep deprivation to the equation, and the risk is further exacerbated. The normal WOCL occurs between 0300 and 0500 body clock time. The NASA study found that for day sleepers there is a circadian shift of approximately three hours. Applying this three-hour change to our circadian clock produces a shifted WOCL between 0600 and 0800 after our first morning arrival. Typically passing our sixth hour of critical period duty (4+30 initial block out to final block in) places us squarely in the shifted WOCL.

2. I learned about circadian sleep patterns and the importance of trying to minimize my sleep deficit as much as possible. Flying on the backside of the clock (and sometimes through the WOCL) is a major part of our job. For most of us, there is no way to avoid this diminished performance zone. The major factors in the fatigue equation are circadian cycle, sleep deficit, and workload. For the most part, we only have control of the sleep deficit portion of the fatigue equation. Sleep deficit can be further broken down into accumulated sleep debt and recent sleep (hours of sleep in the preceding 24 hours).

Most day sleepers cannot achieve the required 7–9 hours of daily sleep in one sleep event. The average first sleep episode was 5.4 hours. Depending on your sleep requirements, an additional 1.6–3.6 hours of sleep per day are needed to maintain your 7–9 hours goal. Every time we miss our daily sleep target, we accumulate sleep debt—7–9 hours of accumulated sleep debt equals 24 hours awake in terms of performance degradation. We do not start recovering from accumulated sleep debt until there is a rest opportunity long enough to accommodate normal circadian sleep cycles.

Unlike sleep debt, recent sleep may be a short-term issue depending on our sleep opportunities. For example, our first a.m. outbound leg will result in less than 7–9 hours recent sleep during that first arrival phase, but as we shift our sleep pattern we can often maintain a rolling 7–9 hours in 24 for the rest of the trips/sequences. Performance begins to decline significantly after approximately 16 hours awake regardless of circadian cycle. This finding makes sense as we begin to infringe on our rolling 7–9 in 24 after the 15th hour awake. Naps, when available, tend to bolster our recent sleep and are critical in combating the circadian sleep pressure we experience on most arrivals. Even a nap as short as 30 minutes temporarily increases our alertness. In my experience, I have found that a nap just prior to critical duty often delays my nodding off on descent/final to nodding off on the way to the hotel.

3. The quality of day sleep is not as good as night sleep within the normal circadian cycle (2300–0800). Even after maintaining 7–9 hours daily sleep, one can expect to feel less alert after several consecutive nights of flying. In my case, the first and fifth nights are the most fatiguing. As an aside, this fact may explain the FAR 117 limit of five consecutive night turns even with the required minimum sleep opportunity of two hours nightly in a sleep room.

4. There are times in the circadian cycle when it is difficult to fall asleep or remain asleep. There is a Secondary Window of Circadian Low or SWOCL (1500–1700), a Circadian Wake Up Signal (1000–1200) and an Evening Wake Maintenance Zone (2000–2200). As a result of a high sleep deficit, many of us will be able to fall asleep in the hub despite being in the 2300–0100 shifted wake maintenance zone.

5. In my research I discovered that our subjective assessment of our fatigue level actually becomes more unreliable as our actual fatigue level increases beyond a certain threshold (saturation). I certainly can vouch for that fact. In my fatigue events I do not recall feeling too tired to start my duty period. I would fall off the “fatigue cliff” at some point during the flight. My most memorable instances involved two micro-naps (uncontrollable short-duration sleep episodes) on final approach. After surviving each instance of extreme fatigue, I would think, “Never again.” Yet, out of ignorance, for years I engaged in what Einstein would term “insanity.” I actually thought that each instance of fatigue was different and that, if I was careful, I would be able to predict my next fatigue episode. Certainly I was able to accurately pinpoint specific trips or combination of trips that I needed to avoid, but accurately judging my limit eluded me.

I learned that I have to make my assessment and decision before excessive fatigue clouds my judgment. Waiting until I fall off the fatigue cliff while airborne was no longer acceptable to me. Through trial and error I figured out what my individual sleep requirements and associated critical duty limits were (micro-naps on final would be considered a clue) and applied them to schedule selection and also to my self-assessment process.

Fatigued individuals can be 50 times more likely to make an error depending on the complexity of the tasks. The unfortunate (or perhaps fortunate) issue with fatigue impairment is that routine muscle memory operations are least affected. It is in the abnormal or adverse situation requiring rapid recognition and reaction where the effects of fatigue are most evident. This fact can lull us into a false sense of security as we successfully complete hundreds of excessively fatiguing legs without significant incidents.

Since fatigue tolerances vary between individuals, we first have to find our personal limit in order to make our fitness-for-duty decision. Once we know our limit, we can monitor our sleep debt and recent sleep. In addition, we can watch for symptoms of fatigue:

  • Forgetfulness
  • Poor decisions
  • Slowed reaction time
  • Poor communication
  • Fixation
  • Apathy
  • Lethargy
  • Moodiness
  • Nodding off

The final decision on safety and fatigue rests with each of us.

Fly Safe,

Don Daley
Captain B-757
Fatigue Risk Management Committee

*Click here to view FAR 117

Textcaster Signup Email Update Contact Update Website Feedback