Under the Influence

Unlike many relatively incomprehensible FARs, the one dealing with alcohol use is reasonably clear.

FAR 91.17 prohibits any person from acting or attempting to act as a crewmember of a civil aircraft: 1) Within eight hours after the consumption of any alcoholic beverage; 2) While under the influence of alcohol; 3) While using any drug that affects the person’s facilities in any way contrary to safety; or 4) While having an alcohol concentration of 0.04 or greater in a blood or breath specimen. Alcohol concentration means grams of alcohol per deciliter of blood, or grams of alcohol per 210 liters of breath.

The regulation goes on to describe when you, as an aircraft crewmember, must submit to a blood or breath alcohol test, and that the results of any such tests may be used in any legal proceedings. These include proceedings to evaluate the pilot’s suitability to maintain his or her pilot certificate.

Clearly, the FAA is concerned with the sobriety of pilots, and flying under the influence can lead to direct action regarding your pilot certificate. It is also interested in patterns of alcohol abuse and this is something that is evaluated during your FAA medical examination. In this article, I will discuss how much alcohol is too much according to the FAA standards.

Complying with the eight-hour bottle to throttle rule is easy, but how likely is it that you will meet the 0.04 standard eight hours after your last drink? That depends on two factors; how much alcohol you ingest and how rapidly your body metabolizes alcohol.

In terms of how much you ingest, let’s use the “standard drink” as a baseline. This drink contains 14 grams of pure alcohol. In practice, this translates to 12 ounces of beer, five ounces of wine or 1.5 ounces of an 80 proof spirit. These standards are somewhat arbitrary and assume beer to be five percent alcohol by volume and wine to be 12 percent. Many modern versions of these beverages have a slightly higher alcohol content.

Ethyl alcohol, the alcohol found in alcoholic beverages, is absorbed into the bloodstream directly from the stomach and small intestine. Absorption occurs fairly rapidly and as a rule of thumb, one standard drink will raise the blood alcohol level by 0.02 percent. This can take as little as 20-40 minutes. Rates of absorption do vary with the weight of the drinker and the amount of food in the stomach. You absorb alcohol faster if your stomach is empty. Blood alcohol concentration can also vary with the sex and size of the individual drinker. Generally speaking, the same amount of alcohol will raise the blood alcohol level more in a smaller person than a larger one, and more in a female than a male.

Another useful rule of thumb is that alcohol is metabolized at a rate of about 0.016 percent per hour. This rate of metabolism is relatively constant among all individuals, but can be slower in persons with liver damage who don’t produce normal amounts of the enzyme that is responsible for alcohol breakdown, or persons who, for genetic reasons, produce slightly different and less effective versions of those enzymes. Another way to look at this is to understand that it takes a little over an hour to metabolize one standard drink. So if you drink more than one standard drink per hour, your blood alcohol level will rise faster than your body’s metabolism can lower it.

Figure 1 shows the blood alcohol concentrations at various times after ingestion of one, two, three or four drinks over a one-hour period. The results are given in mg percentage. (To convert to BAC concentrations, divide by 1,000 so a concentration of 20 mg percent is equal to a BAC of 0.02.) Notice that it took over four hours for the subjects who had four drinks in rapid succession to get back to the 0.04 FAA standard.  Had the person continued to drink over the next few hours, it is quite possible that it would take longer than eight hours from the last drink for their blood alcohol level to drop to below 0.04. In fact, if you had nine drinks over a four-hour period, you would be above the FAA limit eight hours after the last drink. Admittedly that’s a lot of drinking, but remember that the FAR also prohibits acting as a crew member when under the influence of alcohol. That could be interpreted in many ways including suffering from a bad hangover that was impairing your performance.

It’s clearly important to limit your alcohol intake when a flight is on the horizon. Not only should you consider modifying the eight-hour rule to 12 hours, but you should limit your intake so that all the alcohol is out of your system several hours prior to acting as a pilot.

The items discussed in this column are related to experiences by Dr. Seckler in his many years as an AME, and made hypothetical for the article. Any information given is general in nature and does not constitute medical advice.