Under what condition can a pilot's certificate be revoked?

A pilot's certificate can be revoked for engaging in illegal drug use because it directly contravenes the standards of conduct and safety that are essential in aviation. The Federal Aviation Administration (FAA) maintains strict regulations regarding substance use, and pilots are held to a high standard to ensure the safety of themselves and others in the aviation environment. Using illegal drugs indicates a lack of compliance with these regulations and can impair a pilot's ability to operate an aircraft safely, thus justifying revocation of their certification.

The other conditions listed do not typically warrant revocation of a pilot's certificate. Flying without a co-pilot, for example, might be a violation of operational standards or specific regulations, but it does not inherently indicate the same level of risk or negligence concerning safety as drug use. Operating under a foreign license may also present regulatory issues, but it can often be resolved through proper certification processes rather than immediate revocation. Finally, not maintaining a personal logbook, while it may indicate a lack of adherence to record-keeping standards, does not pose a direct threat to safety or public interest, making it a less severe infraction in terms of disciplinary action.