For example, the captain starts pulling back and to the right to turn right. First officer initiates a left diving turn. What happens?
It depends on the aircraft. In any aircraft where the two control columns are linked, there's only one input to command the control surfaces. While most smaller aircraft are connected directly through the control surfaces, Boeing publicizes that on the 787 - a fly-by-wire aircraft - the control columns "[a]re cross-linked between pilots to reduce potential confusion."
On Airbus aircraft (and probably others), the controls are not linked: there is no indication on one pilot's controls of the other's position. The computer handles this by adding the two inputs: opposite commands will cancel each other out, while both pilots commanding the same input will increase the total commanded input. Some Airbus aircraft have a feature which makes an announcement of "DUAL INPUT" when there is input on both sidesticks simultaneously. As of January 30, 1997, this was "an installation" (optional feature, I believe) on the A320 and under development for the A340.
Airbus aircraft also have a Priority Takeover button, which lets one pilot take control and have the airplane ignore inputs from the other sidestick. As Farhan noted, good CRM (Crew Resource Management) is also very important to ensure that only one command is given at once.
Some aircraft have linked controls, which can be separated if necessary. On the 767, if sufficient force is applied, the elevators will "split": each control column will control one of the elevators. The pilot who is stronger or better prepared will have more control over the aircraft. There's also the EMB-145 and -135:
The roll input is ... linked at the control column[.] Each yoke independently controls its own aileron -- the CA yoke only flies the left aileron and the FO yoke the right aileron. It is only through the control column linkage that each yoke can control the whole airplane. It is done this way so that if there is a problem with a cable jamming, the control interlock can be disconnected and at least half control regained.
In the case of 727-100 and 747-100 and -200 aircraft, if both pilots were exerting opposite forces on the control column, the stronger pilot would win. In these aircraft the control columns are linked by heavy duty cables or rods. I don't believe it would be possible for human strength to break them.
The cabling feeds into hydraulic valves, which use hydraulic pressure to move the control surfaces. However, the 727-100 has manual reversion, which enables the pilots to move the control surfaces by the cabling alone should all hydraulic pressure be lost. We used to practice this in the simulator. When flaring for the landing both pilots would work together to move the elevator.
In an airplane when two pilots are actively sitting on the controls, only one is actually flying the airplane. The other is normally watching over the actions of the pilot flying the airplane. Hence the terms are:
- Pilot Flying (PF)
- Pilot Not Flying (PNF) or Pilot Monitoring (PM)
They can switch the responsibility of actual flying at will, or if the captain orders as he/she is the legal commander and always responsible for the aircraft. From Wikipedia:
Even when the first officer is the flying pilot, however, the captain remains ultimately responsible for the aircraft, its passengers, and the crew. In typical day-to-day operations, the essential job tasks remain fairly equal.
When the pilots want to switch the responsibility of flying, they exercise the routine known as Positive Exchange of Flight Controls. This can be performed as following:
PF: You have the controls or Your plane PNF: I have the controls or My plane PF: You have the controls or Your plane
PNF: I have the controls or My plane PF: You have the controls or Your plane PNF: I have the controls or My plane
I've read some people doing the first two lines only, although it is not the standard way.
If you are referring to the fact that both pilots are actually applying contradicting control inputs, then obviously it is not a good thing. It can break the control surface, or the cables connecting them (no in case of fly-by-wire) or even can get very deadly.
The captain starts pulling back and to the right to turn right. First officer initiates a left diving turn. What happens?
On some aircraft, the main controls are linked, often mechanically.
So the strongest pilot, or the one with most stamina, wins.
There may potentially be a period of alternating sudden strong deflections of the control surfaces as the two pilots fight for control.
On some aircraft, the main controls are not linked.
There will usually be visual, audible and sometimes tactile indications to both pilots of a conflict, if both pilots are making inputs.
Either pilot can typically press a priority switch to gain control and cause the other pilot's inputs to be ignored.
There may potentially be a period where conflicting pilots are alternately pressing their priority switches.
Column vs sidestick
Either the traditional control-columns or modern sidesticks can, in principle be designed for either linked or independent operation. It happens that there are two major makers who have historically taken different choices for both these aspects but it is possible to have linked-sidesticks or independent-columns.
The operation of the priority switch is shown in a video
A slightly longer video shows how conflicting inputs are acted on and indicated.
The following text was coughed up by the intertubes and appears to be a description of Airbus sidestick operation.
5.2. Dual input detection enhancements
5.2.1. Description As a complement to the active sidestick evaluations, various new features have been developed to improve the crew awareness of dual input situations on the FBW aircraft. The main objective of these features is to provide warnings which will prevent long duration dual input situations. The following visual, aural and tactile cues have been tested with the active participation of pilots from Airbus, airlines (Cathay, DLH, Sabena), unions (ALPA, SNPL, German Cockpit) and Airworthiness Authorities (CEV, CAA, FAA, Transport Canada).
When both sidesticks are deflected simultaneously (for more than 0.5 sec), the CAPT and F/O captions of the Sidestick Priority Light on both glareshields are illuminated flashing in green. As soon as the priority p/b is pressed on either sidestick, the glareshield lights revert to the classical priority configuration (CAPT and arrow or F/O and arrow). The principles of this visual indicator are identical for all FBW aircraft.
A "DUAL INPUT" audio message is triggered when both sidesticks have been simultaneously deflected for a certain time. The timing of the audio message has been adapted on the A320/A321/A31 and A330/A340 families due to the different systems architecture such that, on all cases, the message is triggered after the illumination of the glareshield lights. In this way, there is a degree of sequencing such that the aural warning will only be triggered if the dual input situation is prolonged. The "DUAL INPUT" audio is repeated every 5 sec. It has the lowest priority of all the audio (voice) messages but can be generated simultaneously with any other non-voice audio warnings. When the priority p/b is pressed on either sidestick the warning is canceled except if it has already started (i.e. it cannot be interrupted).
TACTILE CUE (BUZZER)
The sidestick is fitted with a small electrical motor which rotates an unbalanced weight and thus generates a vibration of the sidestick. The level of vibration depends on the rotation speed and on the weight of the rotating mass. When both sidesticks are deflected for a certain time, the buzzer on both sidesticks are activated to produce a series of intermittent vibrations. Since the level of buzzer vibration could not be increased at will due to various reasons (e.g. rattling noise on the lateral console) the principle of intermittent "shots" was selected to improve the detection of the buzzer activation. The timing of the buzzer activation has been adapted on all the FBW aircraft such that the buzzer is activated simultaneously or slightly after the illumination of the glareshield lights. When the priority p/b is pressed on either sidestick the buzzer is stopped immediately.