Living with the limitations

Reading the Vne placard

The Vne placard is entered with today's pressure altitude and outside air temperature and, where the RFM says so, mass and CG. The headline figure — 155 KIAS power-on at zero pressure altitude in the B3's type certificate, falling about 3 kt per 1,000 ft — is the most generous corner of that table, not today's number. Power-off Vne is lower.

Read it before flight for the highest and warmest part of the route, and again when conditions change. For values between the printed ones, use the method the RFM gives.

Placards can also be added: in 2023, after a fin-spar crack, an EASA AD temporarily reduced the AS355 twin's Vne by placard. An AD can change a number you thought you knew.

Sources 1, 3, 4

Other airspeed limits

Several configurations bring their own speed limits:

• doors off or open — airflow and structural loads
• external load — load stability, and the line
• hydraulics off — a speed range in which forces are manageable
• airworthiness directives — an AD can impose a temporary limit, as EASA's did for B3e tail rotors in 2012

Each comes from a specific RFM section or supplement.

Sources 5, 6

Rotor rpm: two ranges

The rotor rpm indicator carries two kinds of range. Power-on is the narrow band the governor holds while the engine is driving. Power-off is the wider band allowed in autorotation, where you control rpm with collective and flare.

Too low and the blades cone up and lose lift; too high and the blades, hub and gearbox are over-stressed. On the B2, BA and B3 the type certificate lists an audio warning at both ends, and Airbus's H125 specification a high and low rpm warning; the B, B1 and D warn of low rpm only.

Sources 1, 3, 7, 8

Rotor rpm and the horn

Because it marks both ends of the range, the horn can mean opposite things:

• Low NR — the rotor is losing energy: a power loss not yet met with collective, an engine-control fault, or overpitching at high mass and altitude. Recovering rpm needs the collective down, and that costs height.
• High NR — most often in autorotation, where higher mass, higher density altitude and the flare all drive rpm up; some engine-control failures can do it with power on.

The horn says NR is wrong, not why. Confirm on the NR indication and the engine before you act, and fly the RFM's procedure for what you find.

Sources 1, 3, 7, 8, 9

Engine limits: continuous and transient

The engine has limits on torque, turbine outlet temperature (TOT) and gas-generator speed (Ng), each with a ladder of ratings:

• Maximum continuous — usable without a time limit.
• Take-off — higher, allowed for a limited time, for lift-off, hover work and the climb.
• Transient — brief overshoots, usually in rapid power changes, allowed for seconds.

The engine's own type certificate sets what the engine may do; the RFM sets what this installation may use, which can be less. Excursions are recorded, and anything beyond the permitted transient is an exceedance: the RFM and maintenance documents decide whether the aircraft may fly again before an inspection.

Sources 1, 3, 10, 11

The FLI: one needle, three limits

On VEMD aircraft the First Limit Indicator compares torque, turbine temperature and gas-generator speed with their own limits and shows only the one closest, on a single scale with take-off and continuous marks.

That makes the scan quicker and hides what is driving it. Low and cold, torque tends to come first; hot and high, temperature or Ng usually take over. The VEMD will show you which.

The scale is software. Airbus's 2020 H125 power increase was delivered as a VEMD software change — the same engine, more of its power made available. Know which software your aircraft has, and which RFM goes with it.

Sources 8, 11, 12

Mass and centre of gravity

The RFM gives the CG limits as an envelope — CG position against mass, both longitudinal and lateral.

Longitudinal limits keep enough cyclic travel to control the disc. Too far forward and you may run out of aft cyclic to flare or stop; too far aft and forward cyclic runs short.

Lateral CG deserves more attention than it usually gets: the pilot sits on the right, and hook work, hoists, cameras and doors-off loading all move mass sideways. Fuel burn moves the CG too, so check the landing condition as well as the take-off.

Sources 3, 13

Altitude and temperature

The RFM sets a maximum operating altitude, may set separate limits for take-off and landing, and gives an outside-temperature range that can vary with altitude. The B3's type certificate gives an en-route limit of 23,000 ft pressure altitude — 20,000 ft for some modification states.

These are permissions, not promises. Whether the aircraft can hover at a given altitude and temperature is a performance question, answered from the charts for today's mass. The limitation says where you may go; the performance chart says what you can do there.

Sources 1, 3, 14

Wind for start and shutdown

Many rotorcraft manuals limit the wind in which the rotor may be started or stopped. The reason is blade sailing: at low rotor rpm the blades have little centrifugal stiffness, and wind can flap them through large angles — in research and accidents, far enough to reach the airframe or someone nearby.

The risk is greatest during run-up and run-down, in gusts, and with wind over an obstacle or from an unfavourable direction. Cyclic position matters too.

Your RFM gives the limit for your aircraft. Park into wind when you can, and keep people clear until the rotor has stopped.

Sources 1, 15

Slope limits

The slope limit is the steepest ground on which landing and lift-off have been shown with enough control margin. Two things run out: the lateral cyclic needed to keep the disc level while the skids are not, and the margin against dynamic rollover as one skid becomes a pivot.

The limit may differ for nose-up, nose-down and cross slopes. A crosswind, high skids or a lateral CG can use up part of the margin before the slope does. Learn your RFM's figures and the direction each applies to.

Sources 7, 16

Doors-off and open-door limits

Flying with a door removed or open changes the airflow around the cabin and towards the tail. Limits typically cover which doors may be removed or opened, in which combinations, the speed for each and sometimes the sideslip permitted.

The reasons: loads on an open door and its hinges, buffeting, and loose items leaving the cabin into the tail rotor. Removing doors also changes the empty mass and lateral CG in the weight and balance.

The RFM, and any supplement for the door configuration, holds the figures.

Sources 1, 13

External-load limits

With a load on the hook, several limits apply at once:

• the hook's rated capacity — 1,400 kg on the B3/H125 (Airbus figure)
• the maximum mass with external load — 2,800 kg (Airbus figure)
• speed limits with a load, often set by the load's stability rather than the aircraft
• the operating rules — in the US, 14 CFR Part 133 and the approvals it requires

None of these is a performance figure. A hook rated for 1,400 kg says nothing about whether the aircraft can hover with that load today.

Sources 11, 17, 18, 19, 20

Kinds of operation

The RFM states the kinds of operation the aircraft is approved for: typically day and night VFR, with further approvals depending on equipment and supplements.

It also states what is prohibited. The B3's type certificate approves VFR day, and VFR night with the required equipment, and refers to the RFM for the rest; the original AS350 B entry adds "non-icing conditions". Check the list rather than assume.

Sources 1, 3