Why don’t airplanes use turn signals like cars do? Would this improve safety for ground operations at uncontrolled airports? Could a similar concept be used to indicate in air maneuvers?
For example: a specific color could denote a right turn, a left turn, a climb or a descent.
At uncontrolled airports, there is rarely enough traffic for signals to be needed, and pilots should (though are not required to) communicate with each other on CTAF—an option that cars do not have.
In the air, if you're close enough to see a turn signal, you are way too close and need to immediately follow the collision avoidance procedure.
More generally, aircraft are a lot more like boats than cars, and boats don't have turn signals either.
I think the points raised in other answers are good, but they miss the essential difference. Cars choose from a discrete set of options, but planes do not.
When you indicate left when driving you are communicating to other traffic that either you are taking the next left turn, or that you're changing lanes (depending on context). In some situations the exact intention may be slightly ambiguous (is he indicating left for that driveway, or the next road?) but there are never more than a handful of possibilities which means that other drivers are able to transform the indication into the future behaviour by interpreting the information in context.
Contrast this with a plane with a hypothetical left indicator: where is it going? Is it going to turn by 2 degrees? 10 degrees? 50 degrees? More? Is it going to turn slow or fast? The indicator can't tell you, and the information in the sky can't tell you either. Add to that the complexities of communicating changes of direction in three dimensional space and you can see that the information communicated by an indicator - even if it was visible in time - wouldn't provide sufficient information to other planes to be useful.
Aircraft have radios with which to communicate their intent. Cars do not. Pilots should be utilizing these radios even at uncontrolled airfields. A good mantra to have is that there are no uncontrolled airfields. Just pilot-controlled airfields. I will communicate my intent on the Common Traffic Advisory Frequency before entering the movement area of any pilot-controlled airfield. That includes before takeoff and after landing taxiing.
Aircraft have position lights, taxi lights, landing lights, anti-collision rotating beacons, anti-collision strobe lights, ground control, ground marshals, tower, TCAS, ADS-B, sterile cockpit rules (and recommendations for Part 91), runway incursion lighting, etc. with the limited traffic at Class G airports, a couple of more lights will not help.
On the ground, any pilot, aircraft or ground crew should remain well clear of an aircraft displaying all of its lights. And, by regulation, no aircraft should be in motion without displaying at least one of its anti-collision lighting systems. Even in the daytime (unless it was certified without a system). This is unlike a car. Cars can drive around in the daytime without headlights or taillights being on (not counting brake lights and turn signals).
In the air, red, white, and green lights are very distinguishable. Especially when close together at the wattage aircraft use. You would have to have high intensity lights to be able to distinguish between other colors like between red, orange and yellow; or between blue and green. When two aircraft are approaching one another at a closing rate between 100 knots and 1000 knots, pilots do not have the luxury of time to decipher light colors. Instead, pilots rely on the items I mentioned above as well as an explicitly spelled out right-of-way directive found in Title 14 of the Code of Federal Regulations Part 91.113.
§91.113 Right-of-way rules: Except water operations.
(a) Inapplicability. This section does not apply to the operation of an aircraft on water.(b) General. When weather conditions permit, regardless of whether an operation is conducted under instrument flight rules or visual flight rules, vigilance shall be maintained by each person operating an aircraft so as to see and avoid other aircraft. When a rule of this section gives another aircraft the right-of-way, the pilot shall give way to that aircraft and may not pass over, under, or ahead of it unless well clear.
(c) In distress. An aircraft in distress has the right-of-way over all other air traffic.
(d) Converging. When aircraft of the same category are converging at approximately the same altitude (except head-on, or nearly so), the aircraft to the other’s right has the right-of-way. If the aircraft are of different categories—
(1) A balloon has the right-of-way over any other category of aircraft;
(2) A glider has the right-of-way over an airship, powered parachute, weight-shift-control aircraft, airplane, or rotorcraft.
(3) An airship has the right-of-way over a powered parachute, weight-shift-control aircraft, airplane, or rotorcraft.
However, an aircraft towing or refueling other aircraft has the right-of-way over all other engine-driven aircraft.
(e) Approaching head-on. When aircraft are approaching each other head-on, or nearly so, each pilot of each aircraft shall alter course to the right.
(f) Overtaking. Each aircraft that is being overtaken has the right-of-way and each pilot of an overtaking aircraft shall alter course to the right to pass well clear.
(g) Landing. Aircraft, while on final approach to land or while landing, have the right-of-way over other aircraft in flight or operating on the surface, except that they shall not take advantage of this rule to force an aircraft off the runway surface which has already landed and is attempting to make way for an aircraft on final approach. When two or more aircraft are approaching an airport for the purpose of landing, the aircraft at the lower altitude has the right-of-way, but it shall not take advantage of this rule to cut in front of another which is on final approach to land or to overtake that aircraft.
Airplanes don't coordinate visually because they can't
The other answers are correct, but they all seem to miss an important point: airplanes barely see each other! Because (and in contrast to e.g. cars):
The difficult part is to spot another airplane. If two slow flying aircrafts (say Cessna 172) fly e.g. head-on, you may have 10-20 seconds to spot each other in reality. Trying to see detailed lights: impossible. But also not necessary: due to that, aviation does not rely on visual signs but uses:
The last point is especially important: compared to a car, when turning left, you have a continuum of 360 degrees (or 180 if we assume you take the shortest turn). Furthermore, you have up and down.
Actually, it is even not uncommon for VFR aircrafts to request radar guidance instead of relying on visuals.
So what you care about is to know that an airplane, many miles away and not possible to see, is heading from x to y at height z. And this communication is done via radio.
There are pretty strict rules on how planes should travel on the ground. They are generally directed by ground control in the tower, and if they come upon another plane there are guidelines to who has right of way, and unlike car drivers, pilots generally know and follow the rules.
Not being in aviation, I would tend to think that the main reason that signals aren't utility for ground taxiing (which seems the most practical\potentially useful place) is that planes movement are generally setup to prevent the short reaction time we regularly have in cars.
"Reasonable" car spacing, and traffic lights, are such that there is often only around 3 seconds of reaction time. This is usually sufficient if the change can be properly identified, because vehicle size and dynamics allow for quick changes (some exceptions apply). Whereas aircraft are primarily designed for flight, and so I would think may not be as setup to be as adept at sharp ground maneuvers compared to even the more challenging vehicles like semi-trucks.
In addition, I would figure there are expectations of complex distractions (flight checklists) for much of the ground travel time on aircraft, and so I'd expect that ample spacing is fundamental.
As such, taxiing doesn't tend to do things like multi-lane traffic or high traffic intersections. And with everything spaced out more, the sudden dangers of traffic slowing to turn or conflicts when moving into new lanes doesn't exist, and turn signals have fairly limited use. Plus there are comparatively few travel ways, with fairly regular movement for most vehicles, such that people know the spots to expect turns and stops.
If anything, maybe brake lights could be of more use, as I'd think planes are more likely to stop in random places than turn in random places (and brake lights would take care of highlighting unexpected stops made to perform a turn).
Although all of these answers are very good (and correct), they are missing an important fact: planes try to go on the most direct routes (unless mountains, or other landmarks that affect flight, are in the way). This means that most planes will be traveling in a straight path, so turns aren't applicable. That too planes are given routes that do not travel close to the same point at the same time (to prevent a collision). This means that contact (besides at airports) is rare.
Aircraft accidents are not caused by "that plane made an unexpected turn", but by "that plane came out of nowhere".
Also, cars operate on a flat surface. There are 3 choices: Left, Right, Straight.
Aircraft maneuver in 3D. How would turn signals express a climbing turn? A loop?
