Basically the title. This could be referenced to US or European rules.
In the US, taxi time can be logged. 14 CFR 61.51 says the pilot must log (emphasis mine):
Total flight time or lesson time.
And 14 CFR 1.1 says (emphasis mine):
Flight time means:
(1) Pilot time that commences when an aircraft moves under its own power for the purpose of flight and ends when the aircraft comes to rest after landing; or
(2) For a glider without self-launch capability, pilot time that commences when the glider is towed for the purpose of flight and ends when the glider comes to rest after landing.
Note that in reality, general aviation pilots (I don't know about airline pilots) usually just log whatever is on the Hobbs meter, which may include some time sitting still on the ramp doing post-startup checks, setting up avionics etc. That might not be strictly correct per the regulations, but absolutely no one cares.
This question is closely related.
Yes, taxi time is considered as pilot hours because even though the airplane is not flying actions by the pilot are required. Basically, the US rule is in FAR 61.51
(e) Logging pilot-in-command flight time.
(1) A sport, recreational, private, commercial, or airline transport pilot may log pilot in command flight time for flights-
(i) When the pilot is the sole manipulator of the controls of an aircraft for which the pilot is rated, or has sport pilot privileges for that category and class of aircraft, if the aircraft class rating is appropriate;
The "sole manipulator of the controls" is the main thing here, and there's 2 interpretations of this for logging time that I know of, one is that you log from engine start to engine shutdown, the other is that you log from when the airplane starts to when it stops moving (ie. brakes off to brakes on). Both scenarios include taxiing.
I can't find the UK regulation but that's pretty much universal from what I've seen.
Yes. 14 CFR 61.51.e.1.i states a sport, private, commercial, airline transport pilot may log flight time whenever the pilot is the sole manipulator of the controls of the aircraft for which the pilot is rated, or has sport pilot privileges for that category and class of aircraft, if the aircraft class rating is appropriate, or when ii) The pilot is the sole occupant of the aircraft. The regs make no distinction between flying the aircraft and ground operations.
In our flying club we charge by TACH not Hobbs. Some people log hobbs time, but I record the actual engine start on my kneeboard, and then the shutdown time. So the flight time is the wall clock time for the flight.
This can come into play with student operations where there are minimum experience requirements. TACH time is based upon rotations of the engine, and Hobbs time is normally based upon the time the oil pressure is up on an engine. Student experience is based upon logged time.
Since not every aircraft has a working Hobbs meter, and I fly all kinds of different aircraft, logging the start time and stop time gives me accurate data for log entries. Practically, motion begins right after start, because the aircraft is pulled away from the tiedown as soon as the oil pressure comes up. Similarly, one normally stops the aircraft, and begins a pre-shutdown checklist, which is normally complete within just a few seconds of engine shutdown.
Yes, in most cases, taxi time is considered to be part of a pilot's flight time.
Pondlife's answer does a very good job of giving the official definition of flight time as found in 14 CFR 1.1. I want to expand on what that definition means practically in terms of what pilots (under FAA jurisdiction) legally may, or legally must, consider flight time.
When is taxi time considered part of flight time?
For the purpose of clarity, I am going to quote §1.11 again:
Flight time means:
(1) Pilot time that commences when an aircraft moves under its own power for the purpose of flight and ends when the aircraft comes to rest after landing;
I am going to make the assumption that any time spent taxiing is time spent with the aircraft moving under its own power.2
When time spent taxing is for the purpose of flight (for example: starting up, taxiing to the runway, and taking off on a flight), this time is considered part of the pilot's flight time. Since that time spent taxiing is time spent with the aircraft moving under its own power for the purpose of flight, it meets the definition of flight time as defined in §1.1.
When time spent taxing is not for the purpose of flight (for example: starting up, taxiing to the fuel pump, and shutting down to fuel the aircraft), this time is not considered part of the pilot's flight time. Though that time spent taxiing is time spent with the aircraft moving under its own power, it is not for the purpose of flight and does not meet the definition of flight time as defined in §1.1.
Why does this matter?
A pilot may want to keep track of flight time for many reasons, whether toward a pilot certificate, a flying job, a better insurance rate, or merely for personal gratification. Having the agreed-upon definition of flight time as given in §1.1 is helpful for all these ends. However, the important reasons are those where keeping track of flight time is legally required.
In several cases, pilots must keep track of flight time by legal requirement. In some cases, this involves logging the flight time; in other cases this means knowing a record of certain types of flight time with no explicit requirement for written record keeping.
A pilot must document and record (or, log, to put it simply) flight time that is used to meet the requirements for a certificate, rating, or flight review. This requirement is given in 14 CFR 61.51. Since §61.51 does not define what flight time actually is, we have to go back to §1.1 for that definition. In this case, the pilot could log the taxi time if he or she chose to use it to meet the requirements for a certificate, for example.
Pilots flying for Air Carriers or certain Commercial Operators may be subject to flight time limitations. For example, pilots flying under §135 are limited to a certain number of hours of commercial flying. See, for example, 14 CFR 135.267 where a pilot member of a two pilot crew would be limited to 10 hours of commercial flying during any 24 consecutive hours (let's call this "10-in-24"). This commercial flying would include flight instruction, commercial flying for compensation under §91, and other §135 flying, etc. In this case, whether the pilot chose to log any of the above flight time or not, flight time spent taxiing for the purpose of commercial flight would—by legal definition—be included toward that 10-in-24. Let me emphasise this, since it is important to this subject: the totality of commercial flight time—including taxiing for the purpose of flight—is by the legal definition of §1.1 considered to be flight time. Furthermore, since such a pilot is prohibited from accepting any flight assignment that would exceed that 10-in-24 (see more on why in 14 CFR 135.263), the pilot must at least be aware of his or her running 24 hr commercial flight time total, if not logging or otherwise documenting it.
If flight time is tracked either according to a Hobbs time that includes ground time (such as one that starts and stops with oil pressure) or according to block to block time including pushback, the pilot should not have to worry about compliance with flight time limitations since these meet or exceed the definition for legal flight time. However, if flight time is tracked according to either a Tach time or a Hobbs time that excludes some or most time on the ground (such as one that starts and stops according to a weight on wheels switch), the pilot is at risk for non-compliance since these do not record the full legal flight time.
1 Note: I have only included that portion of the definition for flight time that is pertinent to this discussion, since gliders without self-launch capability do not taxi.2
2 I don't think I am wrong about this, but if I am, I would love to be set straight and learn something new!
Note: This answer only addresses operations under FAA jurisdiction.
My instructor always taught me to "test the brakes" immediately upon startup, therefore moving the aircraft under it's own power (and stopping again) so that technically, the hobbs time is an accurate loggable time.. Also a good habit to get into regarding testing brakes before getting distracted with the checklist and having the plane move itself accidentally while you're head down..