Height-Velocity Diagram Lookup
Quickly classify a flight condition against the helicopter Height-Velocity diagram (the 'dead man's curve'). Enter altitude AGL and indicated airspeed and the calculator returns whether the point is inside the H-V avoid envelope, with a recommended action. Approximated for three common training and utility types - always verify against the actual POH H-V chart for the specific aircraft.
Calculator inputs and results
Aircraft
Flight condition
H-V diagram envelope per 14 CFR Part 27/29 type certification + FAA-H-8083-21B Ch 11. Approximation for common training/utility types. Always verify against your aircraft POH H-V chart, which accounts for gross weight, density altitude, and skid/wheel configuration.
How this calculator works
The H-V diagram is part of every helicopter's 14 CFR Part 27 or Part 29 type certification. It defines combinations of height AGL and airspeed from which a safe autorotation cannot be reliably performed in the event of engine failure. The chart has two avoid areas: low-altitude/low-airspeed (Zone B) and high-hover (Zone A).
Zone B (low-altitude, low-airspeed): below approximately 350-450 ft AGL at airspeeds below the rotorcraft's recommended autorotation entry speed. From here you have neither sufficient altitude to enter autorotation nor sufficient airspeed to flare. Engine failure typically results in a hard, run-on, or forced touchdown.
Zone A (high hover): above approximately 350-450 ft AGL at airspeeds below ETL (effective translational lift). Engine failure here gives time to enter autorotation but requires nose-down attitude to gain autorotation airspeed. Sustained high-hover is the textbook example of why pilots accelerate to cruise speed during takeoff.
The calculator uses simplified rectangular bounds based on representative type-certificate data. Real H-V curves are non-linear and shift with gross weight, density altitude, and configuration (skids vs floats). Treat results as a coarse classification only - the active POH chart is authoritative.
Default assumptions & sources
Every default value the calculator starts with, the realistic range you'd see in the field, and the source we used to set it.
| Input | Default | Typical range | Source |
|---|---|---|---|
| Robinson R22 low-avoid | below 350 ft @ <53 KIAS | POH H-V chart | Robinson R22 POH Section 5 - Height-Velocity diagram |
| Robinson R44 low-avoid | below 425 ft @ <60 KIAS | POH H-V chart | Robinson R44 POH Section 5 |
| Bell 206 low-avoid | below 450 ft @ <55 KIAS | POH H-V chart | Bell 206 Performance Section - H-V diagram |
| Safe knee altitude | 8-15 ft AGL | Ground reference hover | FAA-H-8083-21B Ch 11 - autorotation recovery |
What's not modeled
The calculator covers the major cost and time line items. These additional factors apply in some cases but aren't included in the estimate:
- Gross weight effect - H-V envelope expands at higher gross weight (less rotor energy reserve)
- Density altitude effect - high DA degrades autorotation entry margins
- Skid vs floats vs wheeled configuration - some H-V charts publish separate curves
- Wind direction relative to autorotation entry - tailwind reduces effective glide
- Pilot reaction time - approximately 1.5-3 seconds before initiating autorotation entry after recognized power loss
Frequently asked questions
What is the H-V diagram (dead man's curve)?
The Height-Velocity diagram is a chart in every helicopter POH that defines altitude-AGL and airspeed combinations from which a safe autorotation cannot be reliably performed in the event of engine failure. It is required by 14 CFR Part 27/29 type certification. The avoid envelope is commonly nicknamed the 'dead man's curve' because operating sustained in those areas leaves no recovery option if the engine quits.
#Why is there a low-altitude avoid zone AND a high-altitude one?
Two different physical problems. At low altitude with low airspeed (Zone B) you don't have enough height to lower the collective, build rotor RPM, and flare before contacting the ground. At high altitude in a hover (Zone A) you have time but no airspeed - you must dive nose-down to accelerate to autorotation airspeed, which costs altitude that may not be available if the engine fails close to the high-hover boundary.
#Can I fly through the avoid zone briefly during takeoff or landing?
Yes - it is impossible to take off or land without transiting through Zone B briefly. The H-V chart is published to identify SUSTAINED operations to avoid, not transient passes. Best practice: minimize time in the avoid envelope, use a maximum-performance takeoff profile that quickly transitions through the knee of the curve, and never sustain a high hover when an alternative is available.
#Does the H-V envelope change with weight?
Yes. At higher gross weight, autorotation entry requires more altitude and more airspeed, so the avoid zone expands. Robinson POHs publish separate H-V curves for several weight ranges. The Bell 206 POH has a single representative chart but notes higher-weight performance penalties separately.
#What about turbine helicopters with engine restart capability?
Turbine engines have different failure modes than reciprocating, and modern FADEC-equipped turbines have very low in-flight failure rates. However, the H-V chart still applies because the response time from recognized power loss to fully developed autorotation does not change based on engine type - you still need altitude and airspeed margin to recover.
#Related guides & tools
This calculator provides estimates only. Actual aircraft performance and regulatory compliance vary by specific aircraft serial number, density altitude, gross weight, equipment installations, and operator's FAA-approved General Operations Manual / OpSpec. Always verify with primary sources: the FAA (faa.gov), 14 CFR (eCFR at ecfr.gov), your aircraft Rotorcraft Flight Manual (RFM) or Pilot Operating Handbook (POH), the relevant FAA Advisory Circular, and NTSB safety studies for the operational profile.