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Private/Commercial — Helicopter
Helicopter Oral Exam Questions: Height-Velocity Diagram, HIGE & HOGE
The height-velocity diagram — the 'dead man's curve' — is a guaranteed oral topic because it connects performance charts to life-and-death judgment: it maps the combinations of height and airspeed from which a safe autorotative landing may not be possible after an engine failure. Examiners pair it with hover performance: HIGE versus HOGE, and whether today's weight and density altitude leave you enough power to do the job you're planning.
Sources: your rotorcraft flight manual's H/V diagram and hover charts (they are aircraft-specific and usually established at particular weight/density-altitude conditions — read the fine print), and the Helicopter Flying Handbook chapters on performance and emergencies.
Reading the curve
The H/V diagram has two shaded regions. The large low-airspeed/low-to-mid-height region (think: hovering at 300 ft) is dangerous because, after an engine failure, you have neither airspeed to flare with nor enough height to establish a steady autorotation before the ground arrives — rotor RPM decays during the reaction and entry, and there isn't enough stored energy to cushion. The smaller high-speed/very-low-height region (fast and low, e.g., 40 ft at cruise speed) is dangerous because the required cyclic flare at that height risks tail-strike and there's no room to rotate to a landing attitude before touching down at speed.
The safe takeoff profile threads between them: accelerate through effective translational lift close to the surface, then climb along the 'knee' of the curve, trading height for airspeed in a combination that always permits autorotation. Know the conditions under which your RFM's curve was drawn (often max gross weight and a stated density altitude) — heavier or higher-DA conditions effectively grow the shaded areas. Also be ready for the honest discussion: some operations (hoist work, external load, pinnacle approaches, photo flights) necessarily enter the shaded region, which is a deliberate, briefed risk acceptance — not a casual one.
HIGE, HOGE, and hover power
Hover in ground effect (HIGE): within about one rotor diameter of the surface, the ground interrupts the downwash pattern — induced flow and rotor tip vortices are reduced, the lift vector tilts less, and the same hover takes less power. Hover out of ground effect (HOGE) requires noticeably more power; your charts give maximum hover weights for each at a given pressure altitude and temperature. The practical pattern: you may be able to hover on the ramp (HIGE) but be unable to hold a 100-ft hover over trees (HOGE) at the same weight — discovering that at the confined area is too late.
Surface quality matters: ground effect is strongest over smooth, hard surfaces and degraded over tall grass, water, or sloping terrain. Wind helps — even a few knots moves the rotor toward translational lift and reduces hover power. The performance-planning habit the examiner wants to see: before any operation requiring OGE hover (pinnacle, confined area, vertical departure), compute HOGE capability from the chart for today's density altitude and weight, and add a power margin check in flight (a power check on approach comparing power required vs available).
Practice questions with answers & rationales
Q1. What does the height-velocity diagram actually show?
Answer: Combinations of height above the surface and airspeed from which a safe autorotative landing following an engine failure may not be possible — established under specific conditions (often max gross weight, specified density altitude) per the RFM. It's avoidance guidance: takeoff/landing profiles and en route operations are planned to remain outside the shaded regions whenever the mission allows. It is not a 'never works' chart — it's a 'don't bet on it' chart.
Q2. Why is hovering at 300 ft with zero airspeed inside the shaded region?
Answer: After a failure there, rotor RPM decays during your reaction time; with no airspeed there's no upflow from forward flight to help drive the rotor or energy to flare with, and 300 ft isn't enough height to nose over, build airspeed, and stabilize an autorotation before impact. You arrive at the ground with low RPM, high descent rate and no flare energy. The escape from the region is airspeed first — which is why profiles accelerate before they climb.
Q3. Why is the high-speed, low-altitude corner shaded too?
Answer: At cruise speed a few feet off the surface, an engine failure requires an immediate cyclic flare to dissipate speed — but at that height a proper flare risks striking the tail, and without flaring you touch down far above safe ground-contact speed before you can establish a landing attitude. The lesson: low-level high-speed flight removes your margin even though you have plenty of 'energy.'
Q4. Explain why hovering in ground effect takes less power.
Answer: Within roughly one rotor diameter of the surface, the ground restricts the downward acceleration of air through the disc: induced velocity drops, the effective AOA of the blades improves, tip vortices shrink, and the lift vector tilts back less — so less induced power is needed for the same thrust. The result is a higher maximum hover weight IGE than OGE on your performance charts.
Q5. You plan a pinnacle approach at high density altitude. What performance homework is required?
Answer: Compute HOGE capability for the arrival weight and density altitude from the RFM chart — a pinnacle commits you to OGE hover power. Verify power available vs required with margin (and an in-flight power check before committing), brief wind (it reduces hover power required and dictates approach direction), plan the approach to permit a go-around path, and know your escape route if power proves marginal. If HOGE isn't there, the answer is to reduce weight (fuel/passengers) or don't go.
Q6. Does ground effect work the same over tall grass or water?
Answer: No — soft, porous, or moving surfaces (tall grass, brush, rough water) absorb or disturb the outflow pattern, weakening ground effect, so hover power required rises compared to a hard, smooth surface. Slopes and pinnacle edges can also spill the cushion. Operationally: expect to need closer-to-HOGE power over such surfaces and check the margin before reducing airspeed below ETL.
Q7. How do weight and density altitude move the H/V picture and your hover charts?
Answer: Higher weight and higher density altitude both increase the power and RPM energy needed for any recovery, effectively expanding the avoid regions of the H/V curve (which was plotted at stated conditions) and lowering the maximum weights at which you can hover IGE and OGE. The disciplined habit: re-run hover performance for every significantly different condition — morning vs afternoon temperatures can change the answer — and brief the takeoff profile accordingly.
Common mistakes to avoid
- Calling the shaded areas 'illegal.' They're avoid regions established under stated conditions — judgment territory, not regulation.
- Forgetting the curve's fine print: it's typically plotted at specified weight and density altitude — your day may be worse.
- Mixing up HIGE/HOGE or claiming ground effect extends several rotor diameters up (it's roughly one).
- Planning vertical-style departures out of confined areas without computing HOGE power first.
- Ignoring wind entirely — examiners expect you to use it: it shrinks hover power required and changes the takeoff profile.
- Being unable to sketch the curve and the safe takeoff profile through the knee. Draw it during the oral — it answers half the follow-ups for you.