AS350 B3e course · Performance and loading
Performance
Density altitude, hover charts and the height-velocity diagram
The essentials
Everything the Astar can lift depends on density altitude — pressure altitude corrected for temperature. Hot, high and heavy each take margin away, and together they take a lot.
The RFM's hover charts tell you whether you can hover in ground effect (HIGE) or out of it (HOGE) at your mass, altitude and temperature. The height-velocity diagram shows the combinations of height and speed from which a safe landing after engine failure is unlikely.
The Astar has a strong reputation up high. The charts, not the reputation, decide each flight.
Density altitude
Thinner air means the rotor needs more pitch, and so more power, for the same lift — and the engine makes less power, because it breathes the same thin air.
Density altitude rises with altitude, with temperature and, to a smaller degree, with humidity. A summer afternoon at a mid-altitude site can be a higher density altitude than a cold morning on a mountain.
Source 1
Pressure altitude and the standard day
Pressure altitude is what the altimeter reads with the standard setting — 29.92 inHg or 1013.25 hPa. It removes the day's weather from the height.
The standard atmosphere is 15 °C at sea level, cooling about 2 °C per 1,000 ft. Density altitude is the pressure altitude at which the standard atmosphere would have today's density. A day warmer than standard puts density altitude above pressure altitude; a colder one, below.
Working out density altitude (illustrative)
| Pressure altitude | 6,000 ft3 |
|---|---|
| Standard temperature there | 15 − (2 × 6) = +3 °C3 |
| Actual temperature | +25 °C, so 22 °C above standard3 |
| Rule of thumb | about 120 ft per °C above standard (a common approximation)1 |
| Density altitude | ≈ 6,000 + 22 × 120 = 8,640 ft1 |
| Cross-check | FAA-P-8740-2's chart: 6,000 ft at 80 °F (+27 °C) ≈ 8,600 ft3 |
In and out of ground effect
Close to the ground, the surface interferes with the rotor's downwash and the rotor needs less power to hover — roughly within one rotor diameter of a firm, level surface. That is in ground effect.
Over long grass, water, a slope or a cliff edge, or higher than about a rotor diameter, you lose it. Plan any task that needs a hover over those surfaces — long-line work, a pinnacle, a hoist — on HOGE performance.
Reading a hover chart
Hover charts are read in a fixed order: enter with pressure altitude, move across to the outside air temperature, then read off the maximum mass at which you can hover — or, on some charts, the power required.
Read them for the worst point of the flight: the hottest part of the day, the highest site, the heaviest moment. The chart assumes an engine making at least its minimum power — which is what the engine power check confirms.
A hover chart, read (fictional numbers — not AS350 data)
What that example tells you
At 2,150 kg on that fictional day you could land and lift off from a firm, level pad, but not hover over long grass, hold a long line or climb vertically out of trees.
The options are the usual ones: take off less fuel or payload, do the job in the cool of the morning, or change the task so it doesn't need an out-of-ground-effect hover. Some RFM charts also carry corrections for equipment or heating; apply them before comparing.
Hot and high: which limit you meet
Low and cool, an Arriel can make more power than the main gearbox accepts, so the limit you meet first is torque. As temperature and altitude rise, the engine makes less, and the first limit becomes turbine temperature or gas-generator speed.
That is why the Astar holds its performance well up high — it has power in reserve at low level — and why, beyond a certain point, every extra degree costs you lift. On the B3 and B3e, the FLI shows whichever limit you are nearest.
The power check and the charts
The hover charts assume an engine that makes at least its minimum guaranteed power. The engine power check — run on the VEMD on the B3 and B3e, or by the method in the RFM on other variants — tells you how your engine compares.
A positive margin means the charts are, if anything, conservative for your engine. A margin at or below zero means they are optimistic, and the aircraft will run out of power before the chart says it should.
Wind
A headwind gives translational lift without ground speed, so it can make a hover possible that a chart says is marginal. But wind is not dependable: it drops, gusts and changes direction — and a tailwind does the opposite.
Plan on the charts' still-air figures. Treat the wind as margin you might not get.
Wind direction and the tail rotor
Power to hover is only half of it. Wind from some directions also asks more of the tail rotor, or makes its thrust less predictable — and the worst directions on a clockwise rotor are the mirror image of those taught for American helicopters.
Near maximum power, pedal margin can run out before torque does. The LTE chapter shows which directions to respect on the Astar.
The height-velocity diagram
After an engine failure, a helicopter needs either height or speed to convert into a safe autorotation and landing. The height-velocity diagram shades the combinations where it has too little of both.
Low and slow in the hover band, or low and fast near the ground, are the classic shaded areas. Some work — long-line, hoisting, wire patrol — lives inside them by necessity. That is a risk to be managed and briefed, not ignored.
Reading the H-V diagram
The low-speed area runs from just above the ground up to the height at which a vertical autorotation becomes possible. Its upper edge narrows as speed increases: the more airspeed you have, the less height you need.
The high-speed area sits close to the ground: fast and low, there is no time to react and flare before the skids touch. Between them is a corridor the take-off profile is designed to follow.
Source 1
What the diagram assumes
The diagram is established in flight test, on a smooth, hard surface, by test pilots, with a stated mass and density altitude. Some manufacturers publish one diagram for a range of conditions; others vary it. Read the conditions printed on yours.
Real life is worse: rough terrain, a heavier aircraft, a surprised pilot. Treat the shaded edge as optimistic, not as a line you can sit just outside.
Source 1
Working inside the shaded area
A long-line hover at a hundred feet, a hoist, a slow power-line patrol: all sit in the shaded area, and the work can't be done anywhere else.
What can be managed is the exposure: no longer in the shaded area than the job needs, a healthy engine-power margin, a pre-briefed plan for an engine failure in that position, and a departure that accelerates before it climbs where the site allows.
The Astar up high
Because the Arriel 2D can make more power than the gearbox accepts at low level, the Astar keeps much of its performance as it climbs. Airbus quotes hover out-of-ground-effect figures above 12,000 ft for aircraft in particular configurations.
Those are manufacturer figures at stated masses and conditions. The charts for your aircraft, at your mass and today's temperature, are the ones you fly.
Confined areas and pinnacles
Reconnoitre high, then low: the landing area, the obstacles, the wind, and the way out. Approach into wind, and keep a point before which you can still go around without needing out-of-ground-effect power.
On a pinnacle or ridge, the wind rises on the windward side and sinks on the lee. The lee approach can need more power than any chart predicts, so plan the windward one and treat the top as an out-of-ground-effect hover until you are on it.
Source 6
Which day gives the highest density altitude at the same airfield?
Hot and humid. Temperature has the biggest effect; humidity adds a little.
You need to hover over long grass on a slope. Which chart do you plan on?
HOGE. Long grass and slopes destroy ground effect, so plan as if you're out of it.
What does the shaded area of the height-velocity diagram mean?
Too little height and too little speed to set up an autorotation and land safely.
Pressure altitude 4,000 ft, temperature +27 °C. Roughly what is the density altitude?
Standard at 4,000 ft is +7 °C, so it is 20 °C above standard: 4,000 + 20 × 120 ≈ 6,400 ft.
Low and cool, which limit does the Astar usually reach first?
The engine can make more than the gearbox accepts. Hot and high, temperature or Ng takes over.
Your power check shows a negative margin. What does that mean for the hover charts?
The charts assume a minimum engine. Below it, the aircraft runs out of power before the chart says.
Sources
- Helicopter Flying Handbook (FAA-H-8083-21B) · FAA · Chapter 7, Helicopter Performance
- More power and enhanced aerial work capabilities for the H125 · Airbus Helicopters · January 2020
- Density Altitude (FAA-P-8740-2) · FAA
- Helicopter Flying Handbook (FAA-H-8083-21B) · FAA · Chapter 2, Aerodynamics of Flight
- Arriel 2D — Airbus H125 engine · Safran Helicopter Engines
- Helicopter Flying Handbook (FAA-H-8083-21B) · FAA · Chapter 10, Advanced Flight Maneuvers
- Information Notice 2976-I-76 · Airbus Helicopters · Revision 0, 23 February 2016
- AC 90-95, Unanticipated Right Yaw in Helicopters · FAA · 26 December 1995
- Clockwise-rotating helicopter differences · Helicopter Ground
- Helicopter Flying Handbook (FAA-H-8083-21B) · FAA · Chapter 11, Helicopter Emergencies and Hazards
- AC 133-1B, Rotorcraft External-Load Operations · FAA
- 14 CFR 1.1, General definitions (Category A) · U.S. eCFR · Category A, transport category rotorcraft
Educational only. Not approved training material, and never a substitute for the Rotorcraft Flight Manual, an instructor, or your operator's procedures. Figures marked as manufacturer data or test results are not limitations. This content is a draft and has not yet been reviewed by an instructor.