Dead Reckoning and Flight Planning
What this domain tests
Dead reckoning (DR) is position derived from a known fix plus heading, TAS, wind and elapsed time. The exam tests whether you can build and read a navigation log leg by leg, move correctly between IAS, TAS and groundspeed, and judge how quickly a DR position degrades.
Core rules
- DR inputs: last fix, heading (true, magnetic or compass), TAS (from IAS and temperature/altitude), wind velocity, time. Outputs: track made good, groundspeed, present position.
- The 1-in-60 rule: 1 nm of lateral displacement per 60 nm flown is 1 degree of angular error. It converts track error, closing angle and drift into headings.
- Off-track correction angle = track error + closing angle; apply the sum to the heading that produced the error.
- Leg arithmetic: distance / groundspeed = time; time x fuel flow = fuel; add taxi, approach and fixed reserve figures before comparing to endurance.
- Every fix resets DR. Position uncertainty grows with time airborne and with any departure from the planned wind, so plan fixes at intervals that keep uncertainty inside the required containment.
Worked example
Leg 240 nm, TAS 120 kt, wind 270/30, track 090T. The wind is 180 degrees off track, so it is a pure 30 kt headwind: groundspeed 90 kt. Time = 240 / 90 = 2.667 h = 2 h 40 min. At 40 L/h the leg burns 2.667 x 40 = 107 L. With no crosswind component the drift is nil and heading equals 090T; the only correction needed is the variation and deviation chain to reach compass heading.
Traps that cost marks
Using IAS as TAS; forgetting that wind changes groundspeed as well as heading; applying variation and deviation in the wrong order or sign; rounding 2 h 40 min to 2.4 h; and treating a DR position as a fix when answering a question about containment or fuel criticality.