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For buses, it’s quite common to have plastic foil with advertisements on them, but I’ve never seen that on an aircraft. Looking at how aggressively some airlines try to save money, they probably already came up with the idea to sell the space on the side of their aircraft for advertisement purposes – so why did they not do it? Is there any technical reason to not have temporary liveries?

They do have temporary liveries. This is very common in wet lease operators, where an aircraft is flown for an airline during a peak period or when their planes are down for maintenance. Airlines want customers to feel they are getting the brand-name product they are paying for; they don't want customers to board a generic white jet.

An airplane will be painted and decals applied only for several months. Just like motor vehicles, 3M makes aircraft film wraps, including models specifically for short-term use.

It is also common to have a livery for a special event, say the Olympics. You can expect ANA's jet for the 2020 Tokyo Olympics to be returned to normal shortly after the Olympics are over. You can see Lufthansa and Egyptair dressed their planes up for the World Cup. Egyptair's photographic livery can only be done by printing onto a decal.

This video shows Alaska Airlines paint essentially an ad for Disneyland and the Cars movie franchise. They paint the simpler parts like the clouds and the large solid areas, but at 1:28 you can see them apply decals of the characters' faces. They say it took 29 days to paint. This is another video where they're simply applying a decal to advertise another movie.

So, no, there is no technical reason not to have a temporary livery and such schemes are done regularly. It's merely an issue of marketing and customer impressions.

The time needed to apply and remove the advertisements would cost too much money. As long as the aircraft is on the ground, it cannot earn revenue. Better to forgo the small profits from ads for larger profits from operating the aircraft.

1) For a typical urban bus, that plastic foil has to stay on at maybe a max of around 60 mph. For a commercial jet, it'd have to stay on at 600 mph. And suppose it starts ripping off and gets tangled in the elevator...

2) You could repaint the plane at a cost of $50-200K How much does it cost to give an airliner a fresh coat of paint? but the ad would only be seen by the small number of people at the airports, so hardly cost-effective.

Building upon jamesqf answer, I'd like to add:

3) The number of people seeing bus on the streets while moving between stops could be simplified as sum of traffic size + population working and living along its route. For every bus you get a very rough estimate of a thousand potential advertisement targets every minute. And who sees the plane? Only its passengers when they're boarding. People waving their goodbyes usually are too far to read the advertisement, not to mention ppl seeing the plane inflight. That amounts to one to several hundred advertisement targets per several hours, or even whole day in case of longer routes.

The increased cost leads to a fraction of market penetration. Totally not worth it.

They have been using temporary liveries for decades. Rock group Yes famously had one produced on film for their US tour in 197x and half way across the Atlantic it started peeling off and flapping towards one of the engines. YouTube for Rick Wakeman telling this story as he does it so well in his inimitable style

Part of the reason is branding. A city bus is a city bus, but an airline wants to advertise its own brand with its own livery. Some people dislike ads, so that could create a negative perception of the airline.

Part of the reason is cost. An airplane is much larger and more expensive to have out of service.

There's also lower benefit. Buses with advertisements spend most of their time in the city and on roads, surrounded by people they can advertise to. Airplanes spend most of their time in the air, where nobody else gets close enough to see them very well for very long. They do get some exposure near the airport as they are landing or departing, and to people in the terminals.

However, some airlines do put advertising on their planes. It's usually something with a connection to a flag carrier's country, as with Air New Zealand and The Hobbit, or a partner of the airline. Airlines will commonly have some aircraft painted in a livery of their alliance. ANA has had many special liveries, such as Star Wars and Pokemon. The airline tries to make it as appealing as possible, because in addition to advertising something, they are also drawing attention to their own brand.

There are certainly outliers, though. Ryanair is famous for eking out every bit of revenue that they can come up with, and advertise throughout the cabin. They have a history of advertising on their planes, and did announce in 2013 they would be selling advertising space on their planes, but I couldn't find any recent examples other than partners.