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The most obvious one is fuel. Many modern military aircraft, however, are capable of refueling in mid-air. Crew fatigue, another obvious one, can also be addressed by rotating a crew out, and is a non-issue in unmanned aircraft.

The question therefore is, what factors would prevent an aircraft from staying in mid-air indefinitely? Are there any such factors that cannot be addressed, and would make it impossible to design an aircraft that could stay up indefinitely, or at least for an exceptionally long time?

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    $\begingroup$ Crew fatigue can also be addressed with dextroamphetamine, the so-called "go pills" in use by the military. $\endgroup$
    – Terry
    Commented Mar 30, 2014 at 1:10
  • $\begingroup$ Ouch. I know I don't like this comment, but it is truthful. $\endgroup$
    – CGCampbell
    Commented Jun 27, 2014 at 16:03
  • $\begingroup$ Note that we can't even build bridges that stay "up" indefinitely without constant maintenance. And we haven't quite yet mastered mid-air maintenance. $\endgroup$
    – slebetman
    Commented Jul 7, 2014 at 4:12

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Maintenance is probably the biggest issue. What types of maintenance is needed and the failures that not performing those steps will lead to depends on the design of the aircraft.

For example, on a piston engine used in a small aircraft:

The engine will need oil replaced at some point, and unless a system was created to cycle in new oil in-flight this is probably the first thing that will break down after crew and fuel (as you mentioned). Oil breaking down will lead to early failure of the engine.

Fuel and air filters will get clogged, leading to the engine shutting down.

Which leads to engine wear. Eventually piston rings, ignition systems, etc. will wear out.

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    $\begingroup$ You can improve your answer by discussing maintenance on the air frame as well (metal fatigue etc). The only reason I say this is because gliders have become so well designed that some can stay in the air indefinitely because they do not use fuel. Similarly, an electric motor is a very low-maintenance power plant, and with increasingly efficient motors and solar panels, I predict it wont be long (a decade?) before someone makes a self-sustaining solar-powered plane. In this case you'll have to fall back to service intervals for the air frame as well. $\endgroup$
    – MishaP
    Commented Jan 2, 2014 at 23:12
  • $\begingroup$ @MishaP There have already been a few examples of solar powered aircraft being able to stay aloft 24/7. See my answer for more info and examples. $\endgroup$
    – BeowulfNode42
    Commented Mar 28, 2014 at 9:46
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The pilot.

Seriously. You're it.
For the sake of (absurdist) argument, let's say we build something akin to a U-2 out of ultra-strong, super-lightweight composites - let's make it a pure glider (it's close enough anyway, and now we have no engine or anything to get in the way and fail!), and we'll use only the best-traied factory capuchin monkeys to ensure a perfect assembly so all the parts do their job exactly per design.

We've got a million-plus-hour glider, and we just dropped a hapless victim volunteer pilot off at say 50,000 feet with a top-secret life support system that's going to give them heat, air, water, a nice steak dinner every night, and even take care of that odd human desire not to sit in our own waste products.

You could - in this theoretical wonderplane, ride air currents, thermals, storm updrafts (for the crazy), etc. and stay aloft pretty much indefinitely, but eventually our volunteer is going to get sick of steak, or start getting bed sores/blood clots/cramps from the seat, or just plain get sick of looking for that next updraft to keep the glider aloft.

If we make the whole thing remote controlled with a wind-turbine generator to power the controls we could swap out pilots and possibly keep it going for years, but it's not much of an "airplane" if there's nobody flying it -- just another drone.

Ultimately every other factor you can come up with is going to be subordinate to the endurance of the humans on board - as a lot of the GA pilots I hang out with are fond of pointing out many of us fly in aircraft easily have 4-5 hours of fuel endurance, but most of us only have a 2-3 hour bladder (and no magic steak-serving life support machine. Gotta buy me one of those for Christmas...).

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  • $\begingroup$ Pilot fatigue is not a limiting factor for Unmanned Aerial Vehicles. In some cases the UAVs are piloted from bunkers where you can easily change pilots in short intervals indefinitely, use bathrooms, eat, and do whatever humans do. In others, UAVs can hold station completely autonomously without any pilot input. $\endgroup$
    – MishaP
    Commented Mar 28, 2014 at 18:25
  • $\begingroup$ @MishaP Human fatigue is still a factor, even for UAVs (though granted it's much less of a factor - you could have a village of supporting pilots, so you're fine unless a flu epidemic breaks out). Autonomous UAVs push the envelope even further, but they still require some level of monitoring (lest they hold station while a thunderstorm comes through and snaps their wings off). Once the autonomous UAVs are truly 100% autonomous we can then fall back to maintenance and physical factors :-) $\endgroup$
    – voretaq7
    Commented Mar 28, 2014 at 18:36
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    $\begingroup$ Agreed, although at that point I think this question may be changed to "How do we kill that fully autonomous terminator drone bent on exterminating humanity" =). $\endgroup$
    – MishaP
    Commented Mar 28, 2014 at 18:47
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    $\begingroup$ I'm just a layman, but this is pretty surprising. Consider the Volvo Ocean race, where a team of 9 might sail a 65-foot boat for 10 months straight. A large aircraft could certainly provide more comfort than a 65-foot boat. Can any actual plane (not a glider, the question mentioned refueling) really fly for 10 months (50k hours) straight? $\endgroup$
    – raptortech97
    Commented Sep 9, 2014 at 22:52
  • $\begingroup$ @raptortech97 Theoretically Air Force One can be refueled and remain aloft "indefinitely" if we assume 2 or 3 flight crews aboard (subject to the limits of supplies onboard - food, potable water, etc. of course - again, the humans are the limiting factor). I'm sure there would eventually be a mechanical breakdown (or more likely a series of them) that required the plane to land (or crash), but it's conceivable that the plane could fly for months before that happened... $\endgroup$
    – voretaq7
    Commented Sep 10, 2014 at 16:14
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Oddly enough, a malfunction of cabin heat (it was winter) was probably what brought to end the current endurance record holder, a Cessna 172, at 64 days, 22 hours, and 19 minutes (which by the way is a great story to read in itself)

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    $\begingroup$ If you didn't mention this, I was going to. $\endgroup$
    – Steve V.
    Commented Mar 28, 2014 at 15:25
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    $\begingroup$ Wow. Just wow. That's an amazing story. $\endgroup$
    – CGCampbell
    Commented Jun 27, 2014 at 16:25
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The factors and solutions are

  • pilot: there are a few options
    • make it an unmanned aerial vehicle (UAV).
    • and/or make it so it can be intercepted mid flight for crew and supply changes.
    • and/or make it self sufficient. This would require a massive aircraft if it is even possible, certainly not at the moment (2014), though partial self sufficiency could be doable with today's technology.
  • fuel: make it solar. Solar aircraft can store energy as altitude and/or batteries to remain aloft at night. A Solar panel's life is typically considered about 50 years at which time they produce 50% of their original power. There have been a few examples like the NASA Helios that could stay aloft 24/7.
  • bad weather avoidance: has to be fast enough to get out of the way
  • consumables: like oil/grease, etc
    • use non-contact bearings where you can to reduce the need. You can't replace all of them due to the inherent instabilities of non-contact bearings. It is particularly problematic with bearings around high power/load shafts such as thrust bearings responsible for preventing the prop shaft from pulling out of the aircraft.
    • you could also provide systems for replenishing these consumables.
  • moving parts wear:
    • limit the number of moving parts. Electric motors can be built with only 1 moving part
    • provide systems to bring replacements on board and install them with either maintenance droids (yes like the function that R2D2 was supposed to be for), or with maintenance crew that will also need a way on and off the plane.
  • material fatigue such as parts that are developing fatigue fractures and also materials that suffer weather damage (eg plastics/paint degrading due to UV light or metal corrosion): this one is tough as I can't think of anything to replace a critical structural component such as a wing spar in flight. Repairs may be able to be made by temporarily providing alternate structural integrity (think of something like scaffolding but providing structural support around the affected area) while in-situ material repairs are made in the area with damage.

Maintenance is the biggest issue. Titan Aerospace offers UAV aircraft that can stay aloft for 5 years at a time. Basically it can stay in flight until it breaks like the NASA Helios did. Essentially the current thinking is to make it relatively cheap and unmanned while we figure out the average life expectancy of these things so we can bring them in for repair/decommissioning.

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Theory

As voretaq7 notes, the pilots are the first limiting factor here. We can put a submarine crew underwater for 3 months at a time. Could we do the same on a plane? Maybe someday. The International Space Station is an example of what is possible as far as crew rotations. Of course, energy to fly in the atmosphere and external maintenance would pose some problems for an aircraft.

As far as maintenance is concerned, airliners go 500-800 hours between their low level A checks. However, they also undergo routine maintenance at each stop. Mechanical or electronic items that fail are replaced and minor checks are performed. Engines need oil, hydraulic systems need to be replenished.

These are all decisions made as part of the aircraft design process. By making systems more robust, by adding capacity for consumables such as engine oil, these intervals can be extended. Materials such as composites that are less susceptible to corrosion or fatigue can be used. Electric motors need much less maintenance than jet or piston engines, and electricity can come from solar panels.

It really just comes down to the design of a plane. Theoretically, an "indefinite" endurance is certainly possible. It would probably have to be unmanned, but with a robust structural and propulsion design, it could happen. However, everything will fail or wear out eventually, and without a crew or some system to replace components, that will be the limiting factor.

Examples

As far as commercial flights, there is a Wikipedia article that lists the longest scheduled flights by distance, aircraft type, and airline (and also the shortest scheduled flights, just because). The longest duration flight is DAL201, Johannesburg to Atlanta at 16 hours, 55 minutes on a 777-200LR (currently the airliner with the longest range).

With aerial refueling, endurance increases. The B-2 has flown missions of at least 25 hours, with a crew of 2. Landing for a fuel stop (but without engine shutdown), the longest mission is 44 hours. The longest training mission ever was 50 hours. The pilots train extensively in order to be able to fly these missions.

The B-52 has a similar mission and has flown missions as long as 35 hours. Despite being a single seat aircraft, the F-117 has made flights of over 18 hours.

Also notable is the Rutan Voyager, first plane to fly nonstop around the world. The crew of 2 took 9 days to accomplish this.

The Solar Impulse plane is currently in development, and has made a flight of 26 hours with 1 pilot.

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There was infact an experiment to keep aircraft up weeks at a time, using nuclear reactors for energy.

I've heard a few versions, but I believe that Americans never quite got it to work, but the Russians did (i heard sources say that most of their crews got irradiated to death in the process), while I've read other sources neither party got it to work, so I'm quite sure who to believe. In any case- virtually unlimited endurance and range.

nuclear-plane

I guess that would have meant that had they succeeded they could have stayed up for as long as most submarines stay down (which I think is like three months) ... maybe something to try with pilotless drones, where I could imagine that it could stay for pretty longer, engine condition permitting :)

ps. I can't find any good sources on it, but I think that TU Delft was developing a drone for a 365-day mission duration with an environmental surveillance task.

Source 1/Source 2

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