Thermal Stress in the Cockpit
Fighter aircraft are commonly home to locations with stable weather patterns and abundant airspace for training purposes. Many times, these advantages for military flight operations also coincide with very high temperatures. Additionally, military aircraft often deploy to either hot arid desert or high-humidity tropical climates. Unfortunately, heat stress significantly diminishes performance and with prolonged exposure can lead to heat-related illnesses.
Imagine you’re deployed to a hot, desert climate (probably fairly accurate description). You brief a container trailer where the power commonly fails. Today, the AC is limping along. You sweat through a long, complex pre-flight brief that ends 5.1 minutes prior to step. You jog over to AFE and suit up to make your takeoff. After stepping to the jet, you find that your aircraft has been sitting uncovered in the hot sun for hours. It’s 90+ F ambient. You pre-flight your jet and it’s time to taxi. It is naive to think your body temperature will have no effect on tactical performance.
Unique Pilot Risk Factors for Heat Stress
- Required Aircrew Flight Equpiment Layers
- Reduced Air Flow in Cockpit
- Intentional Dehydration to Avoid Urination (you know you’ve done it)
- Radiating heat from avionics and tactical equipment
Humans are ‘warm-blooded’. Unlike reptiles and other ‘cold-blooded’ animals, this means we have the ability (and need to) regulate core temperature within a fairly narrow range, despite the temperature of our environment. Average core body temperature is usually between 35-41 degrees Celsius (95-105.8 Fahrenheit). Heat is gained internally as a by-product of energy production, which obviously increases during exercise. Heat is also gained externally when the ambient temperature around us is greater than the body’s temperature. When overall heat gains exceed losses, core temperature increases. In the opposite setting, core temperature decreases.
The human body has a number of strategies to regulate body temperature. The main way in which the body cools itself is through sweating. In order for sweat to actually translate into heat loss, it must evaporate from the skin’s surface. Therefore, your ability to decrease body temperature will be greatly influenced by the clothing you wear, the presence and speed of wind, and the relative humidity of your environment. Other more minor ways the body loses heat are through shifting blood of to the skin, which allows loss through convection and radiation to the environment, and through behavioral mechanisms (finding shade, taking off clothing, or drinking water).
Your body will likely respond to a gain of heat in a number of predictable ways, which will ultimately decrease performance. This is critical for the professional pilot to consider. As mentioned above, sweating will increase and blood vessels near the skin will open up shifting blood from internal organs (and the cranium!) to the skin’s surface. Although muscular strength does not seem to be affected by excessive heat, both muscular endurance and time to fatigue decreases. One of the most important consequences of heat to a pilot is evidence that heat alone when controlled for fatigue and dehydration, which may confound a study) causes attention, viligance, memory, recall, and decision-making capacity to deteriorate. This has been demonstrated repeatedly in controlled lab experiments1, but an interesting study on Israeli helicopter pilots seems to confirm this also holds true far from the lab, in real-world combat2.
The body’s responses to heat described above likely diminish one’s G-tolerance, though I am unaware of a medical study that specifically answers this question. The act of profuse sweating will certainly lead to fluid loss and dehydration. Dehydration has been linked to lower G-tolerance3. The dilation of blood vessels near the skin surface means less blood in the central blood vessels, and more specifically the vessels that allows you to remain conscious under 9-G’s. Muscular endurance is a necessary to accomplish a strong Anti-G Straining Maneuver (AGSM) and if you are fatigued due to excessive heat on a day in which you trip-turn, your G-tolerance will be pitiful. The RAF’s Aerospace Medicine textbook, Ernsting’s Aviation Medicine, states that the combination of heat, noise, confinement, and vibration diminish G-tolerance by 0.5 to 1.0 G’s.
There are several factors that predispose a person to heat illness or provoke performance deficits from heat. One’s surface area and body mass will cause more blood to shift from the central vessels (negative affect on G-tolerance) and the greater amount of fat tissue acts as an insulating layer preventing efficient heat loss. Sweat response varies somewhat from person-to-person. Although it would seem that heavy sweaters should be able to cool the body more than light sweaters, there is a point where too much sweat saturates the skin, prevents further sweating, and does not easily evaporate. Because sweat has less electrolytes than blood, heavy sweating can lead to electrolyte imbalances as well. Obviously, hydration status is directly linked to sweat production and therefore has an effect on temperature regulation.
GROUND CREW DEMONSTRATING CLASSIC DELIRIUM FROM HEAT-INDUCED ILLNESS
MITIGATING HEAT RELATED PERFORMANCE DEGRADATION
Although cockpit temperatures have been measured in certain instances to be as much as 20 degrees F (11 C) hotter than the corresponding outside environment4, developing true heat illness is not a significant threat to aircrew (though this is a real concern for the ground crew in some locations – see video above). The modern professional fighter pilot should consider the body’s response to temperature in the same way that a professional athlete takes the conversation seriously. When you enter the fight, you need to be performing at your absolute best!
1. Hydration – When you become dehydrated the body’s mechanisms for heat loss are impaired. Dehydration is often quantified as a percentage of body mass. Most studies that have tried to isolate the effects of dehydration on performance have found that cognitive performance is measurably less and mistakes are more frequent starting at 2% dehydration5. As mentioned above, dehydration WILL ALSO worsen your G-tolerance.
- Drink plenty of fluids up until conclusion of preflight brief. This will ensure adequate hydration and enough time to empty the bladder prior to stepping to the jet.
- Keep yourself as cool as possible to minimize sweating. More info below.
- Bring enough fluids to keep you hydrated throughout full duration of sortie(s).
- Thirst alone does not provoke a strong enough impulse to meet the body’s needs for fluids when one is under stress. Try to drink 1 quart of water per hour.
- Gauge the color of your piss – if it’s clear, you’re in the clear.
2. Clothing – Although astronauts and high-altitude flyers have micro-climate cooling systems built into their aircrew flight equipment, most modern fighter pilots do not share this luxury. If you’re wearing an anti-environment (poopie) suit on a warm spring or fall day, know that your ability to control your body heat is close to nil.
- Wear lose fitting clothing where possible.
- Wear natural fibers. Aside from wicking and keeping you cooler, these layers won’t melt to your skin in event of a fire.
- The one downside to the newer full-coverage ATAGS G-suit is the higher burden of thermal stress it causes. Be aware of this.
3. Avoid Additional Heat Stress
- Keep the jet cool preflight.
- Request that jets are parked in the shade.
- Consider having ground crew pre-cool the cockpit
- Minimize long sun-exposed walks to the jet.
- Avoid exercise up to 4 hours prior to flight as this will increase your overall core temperature.
4. Acclimate – The human body is incredibly resilient and adaptable. Your body will acclimate to thermal stress after about 10-14 days of exposure to a hot environment. In order to catalyze this response, you have to be exposed to a minimum of 2 hours of heat per day. This process is further hastened when exercising during heat exposure. Heat adaptation is lost about 1 week after return to cooler environment6.
- If you deploy to the desert or go TDY to the Southwest, intentionally spend the required 2 hours in the sun exercising daily. In 10-14 days, your body should respond considerably better to excessive heat. You have markedly reduced a significant human factor.
- A by-product of acclimatization is increased sweat production at lower temperatures. Understand you will need more water to remain properly hydrated. Gauge the piss color.
- If you leave this environment for a week or more, you’ve started to lose your adaptation. When you return, repeat the process above.
- Unlike heat, the body CANNOT acclimate to a state of dehydration. Avoid dehydration whenever possible.
4. Include Thermal Considerations into Operational Risk Management (ORM) – Understand that heat alone can diminish your performance. In the presence of dehydration, as it usually is, performance degrades further. A thermal stress index was developed in the late 1970’s specifically for flyers of fighter aircraft. The index applies to summer like conditions and does not account for immersion suit or CBRNE protective gear. The index uses an equation involving dry bulb temperature and dew point temperature to provide pilots with one of three helpful advisory categories. This index is called the Fighter Index of Thermal Stress (FITS)7.
- Consult the FITS chart and include the category when determining level of ORM plans. Determine how your flight will mitigate.
- Caution Zone (FITS > 32 C or 89.6 F): Mitigate as directed above. Flight leads and instructor pilots, direct your young wingmen to do the same. Brief to the real risk of flying behind the jet due to combined effects of dehydration and thermal stress.
- Danger Zone FITS > 38 C or 100.4 F): Mitigate aggressively as directed above. Avoid low-altitude missions. Consider canceling sortie if ORM elevated with thermal stress variable.
1. Grether WF. Human performance at elevated environmental temperatures. Aerospace Med. 1983; 44: 747-755.
2. Froom, Caine, Shochat & Ribek ‘Heat Stress and Helicopter Pilot Errors’ Journal of Occupational Medicine Vol 35(7), July 1993, pp. 720-724.
3. Nunneley SA, Stribley RF. Heat and acute dehydration effects on acceleration response in man. J Appl Physiol 1979; 47: 197-200.
4. Nunnley, S A et al ‘Heat Stress in Front and Rear Cockpits of F-4 Aircraft’ Aviation, Space & Environmental Medicine. Vol 52(5) May 1981 pp. 287-290.
5. Gopinathan PM, Pichan G, Sharma VM. Role of dehydration in heat stress-induced variations in mental performance. Arch Environ Health 1988; 43: 15-17.
6. Cheung SS & McLellan TM, ‘Heat Acclimatization, Aerobic Fitness & Hydration Effects on Tolerance during Uncompensable Heat Stress’, Journal of Applied Physiology, Vol 84(5), May 1998, pp1731-1739.
7. Nunnley S A & Stribley R F ‘Fighter Index of Thermal Stress (FITS): Guidance for Hot Weather Aircraft Operations’ Aviation, Space & Environmental Medicine Vol 50 1979, pp.639-642.