High level procedure (Revenue = Price * Quantity) - will give you just the starting point

First clarify perimeter of the market sizing (only passenger fleet or also cargo etc..). What do we include in maintenance (regular manteinance, what is the frequency for a normal aircraft? Extraordinary maintenance e.g. majour overhaul once every xx)

1) Estimate # of aircrafts currently in operations ( Quantity) i.e. the global fleet - I would divide planes that are currently in the sky and planes in airports

You can use both demand and supply slide to solve. Demand = estimate # of aircraft based on how many people are likely to travel in a certain moment. Supply side = based on the capabilities of the infrastructure (airports)

I would use supply side, probably estimate this for a country and then multiply for the ratio population world / population country (first order approximation - you can deep dive and segment nations based on air traffic and so on...).

So for example for US you can use the proxy of # of airports calculated based on number of cities (and states)

e.g. 50 states, 5 major cities per state, one airport in each city

Airports must have a decent traffic so for example if you assume 20 hours/day of operations and landing every 10 minutes (airplane who lands must come from somewhere so you do not double count with airplanes taking off): 6 planes / hour / airport

You know calculate how long the flight requires e.g. 2 hours, you calculate the overall airplane hours = # airplanes total * avg. flight = 6 planes / hour / airport * 2 hours flight = 12 plane hours / airport / hour

Total number of plane hours / hour = 12 plane hours / airport *500 airports = 6000 plane hours / hour

Now the number of aircraft is = 6000 plane hours / hour / 1h = about 6000 aircraft in the us

(Exact number is 7000 aircraft)

Now use an intelligent proxy to extrapolate this to the whole world ( population is not so right, maybe segment in Asia, US etc and give a weight to each continent e.g. asia high)

You should get around something around 20000-40000 aircraft in the world

2) Estimate cost Per single maintenance (€/maintenance)

Consider the labor hours for a major / vs. regular intervention and other cost associated (e.g. materials), quantify them in €/intervention and then decide how many times a year does a airplane goes through this.

e.g. if it is 3 times a year normal maintenance and 1 time major overhaul you do

(3* Normal maintenance cost per single aircraft+ 1 * major overhual cost per single aircraft) / 4 = cost of average maintenance [€/aircraft)

Hope this helps