NDB and ADF
Navigation · Chapter 38 · DGCA CPL and ATPL
How a loop finds a beacon, how to fly with the needle, every common ADF question drawn step by step, and what spoils the bearing.
Remember
- NDB: LF/MF (mostly 250–450 kHz), every direction; ADF: loop + sense = cardioid, one null; switched cardioids ± 5°.
- RB: from the nose, clockwise. QDM = MH + RB; RB = QDM − MH; QDR = QDM ± 180.
- RMI head = QDM, tail = QDR (radial). NDB: variation at the aircraft. A1A needs the BFO.
- Inbound: heading = track − drift, RB = drift. Outbound: RB = 180 + drift. Homing curves into wind.
- Night effect (dawn, dusk), coastal (nearer the coast), quadrantal (045…), dip, Cb. Range 3√P sea, 2√P land.
What are the NDB and the ADF, and why do pilots still use them?
The beacon and the needle
- A beacon that just talks • An ndb is a transmitter on the ground sending the same signal in every direction • Nothing about direction is in the signal: the aircraft works that out itself
- A needle that points at it • The ADF (automatic direction finder) in the aircraft turns a needle to point at the beacon: the RB • Turn the aircraft: the needle moves, the beacon does not
- Why LF and MF • 190–1 750 kHz; most NDBs sit at 250–450 kHz (500 kHz, the old ships’ distress frequency, is kept clear) • Low frequencies travel as a surface wave hugging the ground: a bearing well beyond the horizon, even low down
- Overhead • Straight above the beacon there is no steady signal: a cone of silence, wider the higher you are • The needle swings round as you pass over
What kinds of NDB are there, and where are they used?
Types of NDB
- En route • An en route NDB is a stronger beacon on an airway: 50 NM or more, over oceans several hundred • You fly from one beacon to the next, or use it to check your position • Used on airways, at reporting points and over remote land and sea
- Locator • A low-power beacon near an airfield: only 10–25 NM (locator) • It sits on the approach, in line with the runway, often beside an ILS marker • Used to find the airfield, to hold over and to start the approach
- The difference • Big circle, for the journey. Small circle, for the arrival • Like a motorway sign you can see from far off, and a small sign at the gate • Simple and cheap, so at some small airfields an NDB is still the only approach aid
How does a loop of wire find a direction?
Loop, sense and cardioids
This lesson is part of the Navigation plan. See plans
What is in the aircraft, and how do you hear the ident?
The set, the ident and the BFO
This lesson is part of the Navigation plan. See plans
What exactly is a relative bearing?
What a relative bearing is
This lesson is part of the Navigation plan. See plans
How do you read an RBI?
Reading the RBI
This lesson is part of the Navigation plan. See plans
How do you turn a relative bearing into a QDM?
From RB to QDM
This lesson is part of the Navigation plan. See plans
How does an RMI save you the sum?
Reading the RMI
This lesson is part of the Navigation plan. See plans
Worked questions: read an RMI
RMI: worked examples
This lesson is part of the Navigation plan. See plans
Solve it: read an RMI
This lesson is part of the Navigation plan. See plans
What happens if you just keep the needle on the nose?
Homing
This lesson is part of the Navigation plan. See plans
How do you fly a straight track to an NDB in a crosswind?
Tracking in
This lesson is part of the Navigation plan. See plans
And flying away from it?
Tracking out
This lesson is part of the Navigation plan. See plans
You do not know the wind. How do you find the drift on the way in?
Finding the drift: inbound
This lesson is part of the Navigation plan. See plans
And on the way out?
Finding the drift: outbound
This lesson is part of the Navigation plan. See plans
How does the NDB keep aircraft waiting, and bring them down?
Holding and approaches
This lesson is part of the Navigation plan. See plans
Worked questions: what does the ADF read, tracking in and out, with drift?
The needle with drift
This lesson is part of the Navigation plan. See plans
Track 360°M, 10° drift (wind from the east), NDB abeam right
Beacon abeam
This lesson is part of the Navigation plan. See plans
Radial 200, heading 230°T, variation 7°W. ADF?
Radial and true heading
This lesson is part of the Navigation plan. See plans
RB 316, compass 270, dev 2°W, var 30°E (aircraft), 28°E (station). QUJ?
True bearing to the NDB
This lesson is part of the Navigation plan. See plans
Solve it: QDM from heading and RB, or the other way
Solve it: QDM and RB
This lesson is part of the Navigation plan. See plans
Worked questions: 1 in 60 off track, and time and fuel to the NDB
1 in 60, time and fuel
This lesson is part of the Navigation plan. See plans
Tracking to the station on heading 060°M; asked to track inbound on R100 with a 30° intercept
Intercepting a new radial
This lesson is part of the Navigation plan. See plans
Why can an NDB bearing go wrong at night?
Night effect and other stations
This lesson is part of the Navigation plan. See plans
How can the ground and the sea bend an NDB bearing?
Coast and mountains
This lesson is part of the Navigation plan. See plans
What in the aircraft and the weather spoils the bearing?
The aircraft and the weather
This lesson is part of the Navigation plan. See plans
How far does an NDB reach, and how good is the bearing?
Range and accuracy
This lesson is part of the Navigation plan. See plans
Test yourself on chapter 38
Test yourself
This lesson is part of the Navigation plan. See plans