Members of the Canadian military and the Department of National Defence have been given a not-so-subtle reminder about the optics of attending air shows when the federal government is trying to buy new jet fighters.

A memo — known as a public affairs guidance — was issued Friday reminding everyone of the dos and don'ts while the $19 billion competition is underway.

There is a restriction on attending "any event or activity directly paid for by one of the suppliers," even if it does not relate to the fighter jet procurement. The only way individuals will be allowed to go is if they have advanced "explicit" permission from the department's senior leadership.

It also includes, among other things, a prohibition on talking to the media and the competing companies unless the conversation is given the green light by the commander of the Royal Canadian Air Force (RCAF) — or above.

The directive also said, while attending air shows, the topic of the competition is strictly off-limits, and everyone should be aware of how presence may be interpreted.

"The actions of individual DND employees and [RCAF] members contribute directly to the credibility of our institution as a whole," said the directive, a copy of which was released to CBC News.

"Conflicts of interest in any area of activity can have a negative impact on the perceived objectivity of DND and the CAF, regardless of whether that conflict exists in reality or is simply perceived."

The guidance comes out just after it was revealed the U.S. Air Force F-35 demonstration team will visit Ottawa next month.

The AERO Gatineau-Ottawa air show will host the stealth jet Sept. 6-8, which will be on the eve of this fall's federal election.

The F-35 is one of four warplanes in the $19 billion contest, which was formally launched with a request for proposals by the Liberal government on July 23.

The DND caused a stir in June when it was revealed the Lockheed Martin-built F-35 took part at an air show at the RCAF's principal fighter base in Eastern Canada in Bagotville, Que.

The other competitors to replace the CF-18s — the Boeing Super Hornet, Airbus Military's Eurofighter Typhoon and Saab's Gripen — are not part of the Gatineau-Ottawa air show's program.

The formal competition to replace the Air Force's aging CF-18s was kicked off last month, and bids are not expected to be handed in until early next year.

A decision, though, is not expected until 2022 — meaning defence employees and military members will have to be mindful for years to come.