Smoking has been banned on commercial flights for decades. The signs are lit from pushback to gate. Tampering with a lavatory smoke detector will get you arrested.

And yet every airliner rolling off the line at Toulouse or Renton this year has an ashtray fitted to the lavatory door.

This is not an oversight, or a part nobody bothered to delete from the drawings. It is a legal requirement, and an aircraft cannot be certified without it.

The Regulation Assumes You Will Break the Law

Most safety rules are written on the assumption that people follow the other rules. This one is written on the assumption that they will not.

The logic is simple and slightly bleak. Some passengers, on some flights, will lock themselves in a lavatory and light a cigarette. That is a fact about human beings rather than a hypothesis, and no amount of signage changes it. Given that it will happen, the only useful question is what that person does with the cigarette next.

If there is an ashtray, they stub it out and put it somewhere designed to contain a burning object. If there is not, they panic and drop it into the waste bin, which is full of paper towels.

Two Aircraft Are the Reason

The requirement did not appear because a regulator imagined a risk. It appeared because aircraft burned.

On 11 July 1973, a Varig Boeing 707 on the way from Rio de Janeiro to Paris filled with smoke from a lavatory fire. The crew put the aircraft down in onion fields about five kilometres short of Orly. Relief Captain Antonio Fuzimoto was flying. One hundred and twenty-three people died. Eleven survived.

Ten years later, on 2 June 1983, an Air Canada DC-9 flying Dallas–Toronto–Montreal developed a fire in the air around the rear lavatory. It spread between the outer skin and the interior panels, filled the aircraft with toxic smoke and burned through electrical cables, disabling most of the cockpit instruments. The crew diverted to Cincinnati and landed.

Ninety seconds after the doors opened, the heat inside met the fresh oxygen coming through them and the cabin flashed over. Twenty-three of the forty-six people aboard died — after the aircraft was on the ground, in a survivable landing.

Air Canada 797 is one of the most consequential accidents in aviation history. It produced lavatory smoke detectors, automatic fire extinguishers in lavatory waste bins, fire-blocking seat materials, floor proximity lighting and hard rules on evacuation. The ashtray requirement belongs to the same family of responses.

The Ashtray Is Not the Only Fossil

Once you notice this, the cabin fills up with them.

The lavatory smoke detector exists because of Air Canada 797. The waste bin has its own automatic fire extinguisher for the same reason. The little lock on the lavatory door that cabin crew can open from outside is there because a locked door is a problem during a fire. The seat cushions are made from fire-blocking materials specified after the same accident.

All of these are responses to smoking, and all of them have outlived it. The regulations were never rewritten to remove them, because the reasoning behind each one still holds even in a non-smoking cabin. A fire in a lavatory waste bin does not require a cigarette to start it. Lithium batteries will do it perfectly well, and they are now in every second bag.

Which is the actual answer to the question. The ashtray is not a relic of when smoking was allowed. It is part of the machinery built to survive the fires that smoking caused, and that machinery turned out to be useful for fires that smoking did not cause.

So the next time you notice one, it is not an accident of manufacturing. It is a regulator conceding, in writing, that somebody is eventually going to do the thing they have been told not to do — and making sure the aircraft survives it.

Sources: Federal Aviation Regulations Part 25.853(c); Wikipedia (Air Canada Flight 797, Varig Flight 820); Marketplace; Simple Flying.