11.2b Airframe Symmetry

Question 1

What can misalignment of an aircraft cause ?

Answer 1

Poor flying quality and handling

Question 2

How can misalignment be found ?

Answer 2

By performing a symmetry check and an alignment check using dedicated instruments

Question 3

When must an aircraft symmetry check be performed?

Answer 3

-any structural repair
-when the aircraft has been subjected to severe conditions
-the flight crew reports unusual flight conditions
-wrinkled or buckled structural skins
- loose or sheared fasteners
- areas of badly fitting access and inspection panels

Question 4

what must be done before checking airframe symmetry?

Answer 4

aircraft must be jacked or levelled to ensure datum line is where it should be

Question 5

on small aircraft how is symmetry checked ?

Answer 5

usually have small pegs or blocks that are parallel to the datum lines. a spirit level and straight edge are rested ontop of these

Question 6

other than using a spirit level, what other method can be used to check aircraft symmetry?

Answer 6

the grid method, using a plum bob

Question 7

what are the usual areas that alignment checks are performed?

Answer 7

• wing dihedral angle
  -wing incidence angle
• verticality of the fin
• engine alignment
• a symmetry check
• horizontal stabiliser incidence

Question 8

how can dihedral be checked ?

Answer 8

special dihedral board can should be placed in specified positions.

Question 9

if dihedral boards are not available to check dihedral, what else can be used ?

Answer 9

a straight edge and an inclinometer

Question 10

what can be present when checking dihedral wings ?

Answer 10

dihedral can sometimes be horizontal or on rare occasions anhedral

Question 11

why is incidence checked ?

Answer 11

to ensure the wing is free from twist

Question 12

what can be used to check incidence ?

Answer 12

an incidence board

Question 13

when should fin verticality be checked ?

Answer 13

once the horizontal stabilisers have been rigged correctly

Question 14

how is the fin verticality checked ?

Answer 14

string or a tape measure are strung from prescribed points on the fin and horizontal stabiliser, they must be within prescribed limits

Question 15

how are engines checked for alignment?

Answer 15

method depends on the type of mounting. however generally, a measurement is taken from the centreline of the mounting to the longitudinal centreline of the fuselage

Question 16

where are the specific tolerances, figures and checkpoints for a symmetry check located ?

Answer 16

in the applicable service manual or the AMM

Question 17

how is symmetry checked on a small aircraft ?

Answer 17

steep tapes are used to take measurements between distances, if a long distance is present a spring scale is used to ensure equal tension

Question 18

how is symmetry checked on a large aircraft ?

Answer 18

plum bobs are usually suspended and chalked directly underneath the measurement point. the measurement is then taken from these chalk points

Question 19

why is alignment and symmetry important ?

Answer 19

to ensure the safety and correct performance of the aircraft