Fuel system

Tank to engine

Fuel flows from the tank through a shut-off valve, filters, and the engine's own pumps to the metering unit the engine control commands.

Which electric pumps are fitted, what they do and what happens if one fails varies by variant and modification. Auxiliary tanks fitted under STCs bring their own pump, gauge and flight manual supplement. Learn it for your aircraft from the RFM.

Sources 7, 8

Filters and bypass

Engine fuel filters are typically designed to bypass when they clog, so the engine keeps getting fuel rather than starving. The price is that fuel now reaches the engine unfiltered.

A filter warning, where fitted, means contamination has reached the filter faster than normal — water, debris, microbial growth, or ice crystals in cold fuel. The question it raises is what else is still in the tank. Treat it as the RFM directs, and expect the next sample to tell you something.

Sources 5, 7

Gauges and the low-fuel warning

Older Astars use a resistance-type quantity transmitter with the low-level switch built into it — so the gauge and the warning are not fully independent. Capacitance-type transmitters, and a modification giving a low-level warning independent of the quantity transmitter, were introduced to improve reliability. Airbus's later crash-resistant tank also uses a capacitive transmitter.

Find out which your aircraft has. Either way, if the gauge and the warning disagree, believe the one that gives you less fuel. What the warning means in minutes is in the RFM.

Sources 4, 9, 10

The shut-off lever

On legacy AS350s — B to B2, BA, D and early B3s — the fuel shut-off lever is one of the floor-mounted levers near the base of the collective, beside the rotor-brake lever. Its normal position is down and forward; a snap wire helps keep it there. Later B3s and the EC130 have it overhead.

A lever at floor level in the front of the cabin can be moved by something other than a hand: a strap, a bag, a foot. Airbus has since introduced a modified top plate with an extra detent to hold it stowed (MOD 075101), which EASA recommends but does not mandate.

Sources 11, 12

Crash-resistant fuel systems

The AS350 was certificated before the US crash-resistance rule for helicopter fuel systems (14 CFR 27.952), so its original tank isn't required to meet it.

After the Frisco accident the NTSB recommended crash-resistant systems. Congress then required them for newly manufactured helicopters in the US from 2020. Retrofit tanks are approved for the AS350 family, and many operators have fitted them.

Sources 2, 3, 13, 14

The 1994 rule and its gap

Crash-resistance standards for helicopter fuel systems — §27.952 and its Part 29 twin — became US rules in 1994. They applied to new type certificates only. Helicopters still being built to older type designs, the AS350 among them, didn't have to comply.

The NTSB measured the result: of more than 5,600 helicopters on the US register built since 1994, about 850 (15%) had fuel systems meeting the 1994 standard (registry as of November 2014). Most new helicopters were still being built to pre-1994 designs.

Sources 13, 15

From Frisco to the 2018 law

The NTSB's 2016 recommendations after Frisco asked the FAA to prioritise approval of a crash-resistant retrofit for the AS350 B3e and similar variants, and to tell owners about it and urge them to fit it as soon as practicable.

Section 317 of the FAA Reauthorization Act of 2018 closed the production gap: helicopters manufactured on or after 5 April 2020 must meet the core of the crash-resistance standards, whatever the date of their type certificate. It does not apply to helicopters built before then — for those, a retrofit remains voluntary.

Sources 2, 14, 16

What makes a tank crash-resistant

The aim is to keep fuel inside the system long enough for people to get out.

• A flexible, tear-resistant bladder that deforms instead of splitting.
• Breakaway, self-sealing valves where lines leave the tank, so a torn line doesn't drain it.
• Strengthened or double-skinned lines, a vent system that doesn't spill when the aircraft rolls over, and a flame arrestor.
• A recessed drain that can't be knocked open.

Certification includes a drop test: under §27.952 the tank, 80 % full of water, must fall at least 50 ft onto a hard surface without leaking.

Sources 10, 13, 17

Retrofits for the AS350 family

Two routes exist.

• A tank from Robertson Fuel Systems and StandardAero (formerly Vector), FAA-certified in late 2017 as a replacement for AS350 models from the C and D to the B3e, and the EC130 B4. Air Methods and WeatherTech were launch customers.
• Airbus's own system — a rubber bladder in an aluminium fairing, double-skin lines, new vents and a capacitive gauge — fitted to new H125s and offered for retrofit, with B3 kits from early 2020. Airbus states it meets §27.952 except with an underbelly installation fitted.

Whether a given aircraft has one is in its records and RFM supplements.

Sources 10, 17, 18

Fuels and additives

Turbine engines are approved for a list of kerosene-type fuels, sometimes with alternates or emergency fuels that carry their own limits. The list for your engine, and any restrictions that come with each fuel, is in the RFM and engine documents — not on the fuel truck.

Two additives come up in practice. Anti-icing additive stops water dissolved in the fuel from freezing into crystals that block filters in the cold. Biocides treat the microbial growth that thrives where water sits in a tank. Whether either is required or permitted, and in what concentration, is the RFM's call.

Sources 5, 7