Chasing a 1.15 V Drop on a Fuel Boost Pump Feeder
A 28 V DC fuel boost pump circuit on a transport-category airplane has a 20 ft feeder run (about 40 ft of conductor, round trip) of 10 AWG wire protected by a 20 A breaker, and the pump draws 20 A. A technician chasing an intermittent low-pressure caution measures 1.15 V from the bus terminal to the pump connector pin with the pump running. The run contains two butt splices installed during earlier repairs, both inside a loom in a high-vibration area near the pump. The technician proposes adding a third splice to replace a chafed 6 in. section of conductor, using an insulated butt splice covered with heat-shrink.
1. Does the 1.15 V reading indicate a fault? If so, where is it, and how do the numbers support your answer?
Read the worked answer
Yes, there is a fault, and it is in the connections, not the wire.
- 10 AWG copper is about 1.0 ohm per 1000 ft. For 40 ft of conductor: 0.04 ohm.
- At 20 A: 0.04 x 20 = 0.80 V of drop attributable to the wire itself.
- Measured total is 1.15 V, so roughly 0.35 V is being dropped across the two splices and the pump connector.
A sound splice or terminal at rated current contributes only millivolts (many OEM standard-practice manuals cap it around 50 mV). Losing 350 mV in two splices plus one terminal means at least one connection has high resistance. That connection will heat, can arc, and is a plausible cause of the intermittent caution.
Note the total drop is still under the usual ~2 V allowance for a 28 V circuit, so a technician who only checks the total will pass this circuit. The diagnosis comes from where the drop sits, not the total.
2. How would you isolate which connection is at fault, and what measurement precautions apply?
Read the worked answer
Isolate by loaded voltage-drop testing, one connection at a time.
- Keep the circuit energized at rated load (pump running or a load bank at 20 A) and put the meter probes on the two ends of a single connection - lug body to lug body - never to ground.
- Measure across splice 1, splice 2, and the connector separately. The one showing the large drop is the fault.
- While monitoring, flex and tap the loom; a drop that jumps confirms an intermittent crimp or a broken strand under the insulation.
- Inspect for discoloration, hardened insulation, and heat-shrink that has flowed.
Precautions: do not use the ohmmeter function to judge this; a DMM's ohm source pushes microamps and will read a good high-current joint as bad. De-energize before any physical work. Do not use an insulation tester (megger) on the circuit. Use correct crimp tools and inspect with the pull test the standard practices require.
3. Repair decision: install the third splice as proposed, or replace the run? Justify your call and state what the repair must include.
Read the worked answer
Replace the run.
- The conductor already carries two splices in a high-vibration zone near a rotating machine. Splice count per run is limited by standard practice for exactly this reason: each splice is a new failure point and a stress riser.
- The proposed repair addresses a 6 in. chafed area and leaves two suspect connections in place; the original 0.35 V problem would survive the repair.
- A continuous new wire removes all three unknowns at once.
If replacement is impossible in the field, a splice is permissible only with an approved environmental splice, the correct calibrated crimp tool, staggered placement, support within a few inches of the splice, and 2:1 insulation coverage - followed by a repeat loaded voltage-drop check showing millivolt-level drop across the new joint. Document the splice count in the aircraft records. Route and clamp the new wire per AC 43.13-1B, with chafe protection at every standoff.