Radio altimeter

Question 1

outputs of radio altimeter

Answer 1

TCAS
GPWS
auto pilot

Question 2

normal Radio altimeter frequency

Answer 2

4300Mhz

could be between 4200-4400

Question 3

how does the radio altimeter work

Answer 3

transmits continuously via one antenna and receives via another
cable length from antenna to computer compensated for in calculation
uses continuous wave frequency and modulates it
modulated +/ - 50MHz - so frequency 4300 +/-50Mhz
produces frequency sweep over a time period to provide only one comparison
computer measures frequency difference ,which is proportional to height, at a given time

Question 4

radio altimeter Antenna characteristics

Answer 4

conical beam used so you can pitch and roll and still get a reading
beam width 30-60°
wider in roll than pitch

Question 5

Radio altimeter limitation

Answer 5

does not give height above runway
gives height above the ground directly below the aircraft
for auto land, a flat region prior to the runway is needed ‘radio altimeter operating area’

Question 6

In glide slope

Answer 6

calculation gives absolute height of lowest wheels
datum set to main gear
residual height - between gear and antenna

Question 7

radio altimeter accuracy

Answer 7

+/- 2ft or +/-1.5% below 500ft

gives indications between 0-2500ft- although is always active

Question 8

radio altimeter display

Answer 8

logarithmic
dedicated display or part of PFD
ability to set & display decision height

Question 9

radio altimeter errors

Answer 9

residual height- could give -ve indication when on the ground- not really an issue
failure- usually have 2 for redundancy
fail to zero- is possible for system to erroneously indicate 0ft even though you are still airborne