Australian Flying - July 2018

australianflying.com.au

which their own judgement would

have them attempt. In this respect,

as well as that of the relative

isolation of many survey areas, the

pilot places a great deal of faith in

the mechanical airworthiness of

the survey aircraft. Most survey

traverse lines will dictate an

oblique approach to a ridgeline,

which goes well against safe

mountain f lying technique usually

practiced in other low-level f lying

disciplines. In many cases there is

little margin for safety in the case

of an engine failure. Under these

circumstances, low-level engine

failure procedures should be well

drilled and be held in second

nature by every survey pilot.

The navigation display available

to the survey pilot is usually a

combination of a radar altimeter

for height reference and an

agricultural-type GPS line marking

system consisting of a light bar and

a moving map screen display. It can

take quite some time for the trainee

survey pilot to become accustomed

to the use of both the light bar and

radar altimeter to maintain f light

within survey specification in the

3D plain.

For this reason many airborne

survey companies will favour

pilot recruits who have a history

of agricultural f lying behind

them. Those pilots generally

take little training in order to

reach acceptable f lying standards

for survey as they are already at

home with the light bar and low-

level environment.

Other survey navigation

displays may consist of a single

screen graphical display designed

in an ILS-type fashion giving

both height and cross track error

information within the one display.

Other supplementary information

such as ground speed, distance

to boundary, and numerous

other parameters may also be

displayed on such a screen in a user

configurable fashion.

Flying survey lines for the first

time will be a significant learning

curve for any pilot. Developing the

dexterity of maintaining cross-track

accuracy while simultaneously

maintaining height accuracy in

the 3D plane is a process born

of considerable concentration.

Far from maintaining a constant

descent and cross-track accuracy

while performing an ILS approach

for a few minutes, maintaining

40

AUSTRALIAN FLYING July – August 2018

discretion with regard to the

approach made towards any

significant feature of terrain. The

survey line must be f lown within

a specified tolerance, which

means that height and cross-track

accuracy must be maintained

within an agreed tolerance.

Where rising terrain is likely

to out-climb the performance

capability of the aircraft, a

technique of "draping" significant

terrain features is used. If you

can imagine draping a sheet over

a scale model of the terrain, then

that might be the best description

of the resulting f light path along

all relevant survey lines. Here

height tolerance is stretched in

order to allow for the mechanical

capability of the aircraft.

Flying the lines

The need for cross-track accuracy

remains a constant across the

survey area, which can mean that

a pilot’s approach to a sharp ridge

line will generally be against that

first line in the set that they have

been allocated for that f light. All

the way the pilot is assessing and

reassessing the actual weather. It

is sometimes possible for a survey

area to be located up to 100 nm

away from the base airstrip, so it

pays to be alert to any weather

pattern that might threaten

the aircraft’s access to the base

aerodrome while productively

f lying the survey area. Alternate

aerodromes or landing areas are

not a feature of the interior in

many cases.

The survey lines to be f lown

will be established by client

specification according to their

required resolution of data.

The majority (usually 90%) of

survey lines are those aligned in

one particular direction. These

are known as the traverse lines

and will usually be aligned

perpendicular to the average

line of the local geology. The

remaining 10% of lines are those

referred to as the tie lines, and will

be aligned at right angles to the

traverse lines and at 10 times their

spacing. The data collected from

the tie lines is used to level all that

collected from the traverse lines,

neatening the resolution of that

final data and its resulting image.

Due to the nature of the survey

lines being pre-set by the job

specification, the pilot has no

LEFT: The cockpit of a survey FU-24 Fletcher

showing a now-obsolete survey guidance system.

BELOW: David Trust prepares a PAC

Cresco for deployment to Cambodia.

Survey Flying

“ However, family life can

quickly become a major

casualty whilst leading the

airborne survey lifestyle.”