7/26/2019 Stabilised ROD for Approaches

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Stabilised rates of descent for

non-precision instrument

approaches

C

ARRYING OUT ANY INSTRUMENT

approach, p articularly in single-pilot

operations, requires a high level of

concentration, skill and accuracy.

The Non-Directional Beacon

NDB)

approach is probably the most demanding.

Anything that reduces the workload will

reduce the possibility of an accident.

One method of d oing this is to fly, where pos-

sthle,a stabilised approach. A stabilised approach

is w here the speed and rate of descent are con-

stant throughout the approach.

Once set up, this cuts the workload by

reducing the need for large power changes,

pitch changes and the resulting trim changes.

NDB

approach (shown above). The approach

com m ences at 2,800ft overhead the

CV NDB.

The category A and B outbound leg is 3 m in-

utes, and there is a minimum height of 2,500ft

before you are aligned on 147 inbound to the

NDB.

Flying a 3 minute outbound leg and com-

pleting the turn inbound at rate 1 w ill require

4 m inutes and 4 seconds, all for a 300ft descent

which equates to a stable descent rate of 78ft

per m inute. O n the inbound leg on a track of

147, the descent is 600ft in 3 minutes or a sta-

ble descent of 200ft per minute. To fly a sta-

bilised approach with constant speed and a

constant rate of descent, a higher commence-

ment height will be needed.

For instrument approaches based solely on

a tracking aid, it is impo ssible to accurately

determine that the aircraft is achieving a 3

approach gradient. The maximum rates of

descent which can be specified for the out-

bound and inbound tracks of a procedure are

listed in ICAO DOC

8168 Vol II

PANS OPS).

There are approaches with very shallow

descents which do not lend them selves to a

stabilised approach. The reason is that the

25nm minimum sector altitude is often the

commencement height for the approach.

In the case of C harleville, the approach com-

mences at the 25nm minimum sector. This

m eans that an aircraft arriving at Charleville

in IMCwill maximise the possibility of becom-

ing visual and therefore m ay not require the

NDB

approach and the extra flight time.

If your aircraft is not established in

VMC at

the 25nm m inim um sector altitude, you can

go straight into the instrument approach.

Changing the approach so that stabilised flight

is possible throughout the approach will have

negative effects w hich outw eigh the benefits.

To reconcile the ideal situation with the

practical solution offered by the instrume nt

approach procedure designers, an instrument

approach can be commenced at a height in

excess of what is published on the chart. In

the Charleville approach, if you were certain

that the approach m ust be flow n to becom e

visual, the procedure could be com m enced at

5,400ft in nil wind - allowing a descent rate of

about 500ft per minute.

A stabilised approach offers a reduced w ork-

load. Some approaches may have an initial

approach fix below the height from which a

stabilised approach can be flow n. To use a sta-

bilised approach, w ork out a comm encement

height which allows for the procedure. Ensure

that the heights you select fall within the nor-

mal operating parameters of your aircraft.

Allister Polkinghorne is C S safety education officer

To fly this procedure you m ust establish a pro-

file which lends itself to this method.

The information required is the descent

rate needed to make good the approach angle.

For most

ILS, VOR/DME

and

NDB/DME

approaches, the approach angle is 3, which

can be m atched to the gradient-to-rate table

in the front of the

DAP,

nd is shown on the

DMEaltitude scale.

Procedures

The descent rate varies on

NDB

or

VOR

approaches without distance information. The

descent rate can also vary w ithin the approach.

Take the example of the Charleville RW Y 12

22 FLIGHT SAFETY AUSTRALIA AUTUMN 1997