Image display

1. A method for displaying an image to a pilot of an aircraft, the method comprising:

displaying, by a display, to the pilot of the aircraft, an image comprising guidance symbology for use by the pilot when landing the aircraft, the position of the guidance symbology on the display being dependent upon an output of an Internal Navigation System of the aircraft wherein the display is a helmet mounted display of a helmet for wearing by the pilot;

providing, to one or more processors, an indication that the output of the Internal Navigation System is incorrect;

determining, by the processor, a further value for the output of the Internal Navigation System;

determining, by the one or more processors, a specification of further guidance symbology including a position for the further guidance symbology on the display, wherein the position on the display of the further guidance symbology relative to the position on the display of the guidance symbology is dependent upon the difference between the further value for the output of the Internal Navigation System and the incorrect output of the Internal Navigation System; and

displaying, by the display, to the pilot of the aircraft, an image comprising at least the further guidance symbology.

2. A method according to claim 1 , wherein the step of displaying, by the display, to the pilot of the aircraft, an image comprising the further guidance symbology comprises replacing the guidance symbology by the further guidance symbology.

3. A method according to claim 2 , wherein

the guidance symbology is a first Ship Reference Velocity Vector scale, the position of the first Ship Reference Velocity Vector scale on the display being based on the incorrect output of the Internal Navigation System; and

the further guidance symbology is a second Ship Reference Velocity Vector scale, the position of the second Ship Reference Velocity Vector scale on the display being based on the further value for the output of the Internal Navigation System.

4. A method according to claim 1 , wherein the guidance symbology and the further guidance symbology are linear objects that extend horizontally across at least part of the display.

5. A method according to claim 1 , wherein

the output of the Internal Navigation System is a height rate for the aircraft; and

the further value for output of the Internal Navigation System is a corrected height rate for the aircraft.

6. A method according to claim 5 , wherein the step of providing, to the processor, the indication that the output of the Internal Navigation System is incorrect comprises:

controlling, by the pilot, the aircraft to operate in a straight and level state;

determining that, during the straight and level state, the height rate of the aircraft output by the Internal Navigation System is non-zero; and

responsive to the determination that the height rate is non-zero, providing, to the processor, the indication that the output of the Internal Navigation System is incorrect.

7. A method according to claim 5 , wherein the further value for output of the Internal Navigation System is determined using the following formula:

{dot over (h)} correct =(ν aircraft −ν s )tan γ SRVV

where: {dot over (h)} correct is the corrected height rate for the aircraft;

ν aircraft is the velocity, relative to the Earth, of the aircraft;

ν s is the velocity, relative to the Earth, of a runway upon which the aircraft is to be landed; and

γ SRVV is an angle, relative to the runway, of a desired glide slope for the aircraft.

8. A method according to claim 1 , wherein the determination of the further value for the output of the Internal Navigation System is performed by the processor in response to the processor receiving the indication that the output of the Internal Navigation System is incorrect.

9. A method according to any of claim 1 , wherein the one or more processors is wholly located onboard the aircraft.

10. A method according to any of claim 1 , the method further comprising controlling, by the pilot, the aircraft dependent upon the displayed further guidance symbology.

11. Apparatus for displaying an image to a pilot of an aircraft, the apparatus comprising:

a display configured to display, to the pilot of the aircraft, an image comprising guidance symbology for use by the pilot when landing the aircraft, the position of the guidance symbology on the display being dependent upon an output of an Internal Navigation System of the aircraft; and

one or more processors configured to:

receive an indication that the output of the Internal Navigation System is incorrect;

determine a further value for the output of the Internal Navigation System; and

determine a specification of further guidance symbology including a position for the further guidance symbology on the display, wherein the position on the display of the further guidance symbology relative to the position on the display of the guidance symbology is dependent upon the difference between the further value for output of the Internal Navigation System and the incorrect output of the Internal Navigation System; wherein

the display is further configured to display, to the pilot of the aircraft, an image comprising at least the further guidance symbology.

12. An aircraft comprising apparatus according to claim 11 .

13. A program or plurality of programs disposed in a non-transitory machine readable medium arranged such that when executed by a computer system or one or more processors it/they cause the computer system or the one or more processors to operate in accordance with the method of claim 1 .

14. A non-transatory machine readable storage medium storing said program or at least one of the plurality of programs according to claim 13 .