Night Over Water: GPS Lost, Position Fixed by VOR/DME Only
Night VFR over open water, GPS failure. Aircraft: PA-28-181, TAS 110 kt, fuel 68 L, burn 30 L/h, no icing. The pilot is inbound to VOR ABC on the 215 radial. At 12:00 the DME reads 34 nm; at 12:12 the R-215 is still centred and the DME reads 22 nm. The OBS is set to 215, the tracking indicator is centred, and the aircraft is holding heading 045. The destination airport is 25 nm beyond the VOR along the 035 radial, over water. An alternate is 15 nm east of the VOR, with full lighting. ATC offers an eastward vector for weather avoidance. The panel GPS has not regained a fix for eight minutes and the handheld unit's battery is dead. There is no moon; the sea and sky are indistinguishable. The pilot does not hold an instrument rating and has not flown a VOR approach in six months.
1. Determine the aircraft's position at 12:12 and the ground speed. Show the reasoning.
Read the worked answer
Position
A VOR radial is a line of position and a DME distance is a circle; two of them fix the aircraft. The R-215 is centred and the DME reads 22 nm, so at 12:12 the aircraft is on the 215° radial at 22 nm from ABC, southwest of the station.
Being on the 215 radial and flying inbound with the OBS set to 215 means the aircraft is tracking 035° magnetic toward the station, not away from it. This matters: if the pilot thought he was outbound, every subsequent heading would be wrong by 180°.
Ground speed
Distance run = 34 − 22 = 12 nm. Time = 12 minutes = 0.2 h. GS = 12 / 0.2 = 60 kt.
Time and fuel to the VOR
22 nm at 60 kt = 22 minutes. At 30 L/h that is 11 L, leaving 57 L - still comfortable against the 15 L reserve.
The key discipline here is to write the fix down with the time, because the next fix is the only thing that will tell him what the wind is doing to him.
2. Compute the wind from the tracking information, and explain what it means for the next leg.
Read the worked answer
Known values
- TAS = 110 kt
- Heading = 045°
- Track = 035°
- Ground speed = 60 kt
Components
- Drift: 10° with the nose to the right of track, so the aircraft is being pushed left → wind from the right. Crosswind component ≈ 110 × sin 10° ≈ 19 kt.
- Along-track: 110 × cos 10° ≈ 108 kt of airspeed along track, but the aircraft is achieving 60 kt over the ground → headwind ≈ 48 kt.
Resultant
- Speed = √(19² + 48²) ≈ 52 kt.
- Direction: a pure headwind would be from 035°; the crosswind is from the right of track, i.e. from 035° + 90° = 125°. The wind lies between them, weighted toward the larger component: arctan(19/48) ≈ 22°, so wind from about 057° at 52 kt.
What it means
- The 52 kt wind at 90° to the intended route outbound along the 035 radial means the outbound leg (destination beyond the VOR) will see this as a crosswind plus a component - recalculate, do not assume the same 60 kt.
- A 48 kt headwind on a 110 kt aircraft is a 44 percent loss of range. Any fuel plan based on still-air figures is invalid.
- Tracking accuracy now matters more than speed: over water at night with no horizon, a 10° drift held for 25 nm puts the aircraft several miles off track with no visual cues to correct it.
3. Fuel is 68 L with a 30-minute reserve. Give your routing decision, including how you would use ATC's offer of a vector.
Read the worked answer
Fuel arithmetic
- Reserve 30 min = 15 L.
- Usable for the flight = 68 − 15 = 53 L = 1 h 45 min at 30 L/h.
- To the VOR: 22 min. Remaining after the VOR: about 1 h 20 min of usable fuel.
- VOR to destination: 25 nm. At the present 60 kt that is 25 min; even at 50 kt it is 30 min. Both are within the 1 h 20 min available.
Decision: continue to the VOR, then to destination, with a hard diversion trigger.
- Keep the VOR the primary. Fly the 215 radial inbound; the CDI is the only reliable lateral guidance and the DME gives continuous range. Do not let the failed GPS become the focus.
- Use ATC's vector, but keep it short and defined. A 10–15° eastward deviation for a few miles is acceptable; ask for a heading that returns to the 035 radial and confirm the distance. A long vector over water at night without own navigation is not acceptable.
- Set a decision point at the VOR. If the fuel at the VOR is below 55 L, or if the destination is not reporting VFR, divert to the alternate - 15 nm east with full lighting is a far better night destination than an unlit coastal field.
- Do not fly a VOR approach - the pilot is not instrument rated and is out of practice; a night visual approach to a lit runway is the correct option.
Justification: the fuel allows the destination, the alternate is available throughout, and the only real risk is drifting off track over water with no horizon. Fixing position every few minutes and keeping a fixed diversion trigger converts that risk into a decision already made.