Helicopter Instrument Flight Rule (IFR) Certification - Part 1

It is very important that pilots be familiar with the IFR requirements for their particular helicopter. Within the same make, model, and series of helicopter, variations in the installed avionics may change the required equipment or the level of augmentation for a particular operation. The Automatic Flight Control System/Autopilot/Flight Director (AFCS/AP/FD) equipment installed in IFR helicopters can be very complex. For some helicopters, the AFCS/AP/ FD complexity requires formal training in order for the pilot(s) to obtain and maintain a high level of knowledge of system operation, limitations, failure indications, and reversionary modes. For a helicopter to be certified to conduct operations in instrument meteorological conditions (IMC), it must meet the design and installation requirements of Title 14 Code of Federal Regulations (14 CFR) Part 27, Appendix B (Normal Category) and Part 29, Appendix B (Transport Category), which is in addition to the visual flight rule (VFR) requirements.

These requirements are broken down into the following categories: flight and navigation equipment, miscellaneous requirements, stability, helicopter flight manual limitations, operations specifications, and minimum equipment list (MEL).

Flight and Navigation Equipment

The basic installed flight and navigation equipment for helicopter IFR operations is listed under 14 CFR Part 29, § 29.1303, with amendments and additions in Appendix B of 14 CFR Parts 27 and 29 under which they are certified. The list includes:
  • Clock
  • Airspeed indicator
  • Sensitive altimeter (A “sensitive” altimeter relates to the instrument’s displayed change in altitude over its range. For “Copter” Category (CAT) II operations, the scale must be in 20-foot intervals.) adjustable for barometric pressure.
  • Magnetic direction indicator
  • Free-air temperature indicator
  • Rate-of-climb (vertical speed) indicator
  • Magnetic gyroscopic direction indicator
  • Stand-by bank and pitch (attitude) indicator
  • Non-tumbling gyroscopic bank and pitch (attitude) indicator
  • Speed warning device (if required by 14 CFR Part 29)

Miscellaneous Requirements

  • Overvoltage disconnect
  • Instrument power source indicator
  • Adequate ice protection of IFR systems
  • Alternate static source (single-pilot configuration)
  • Thunderstorm lights (transport category helicopters)

Stabilization and Automatic Flight Control System (AFCS)

Helicopter manufacturers normally use a combination of a stabilization and/or AFCS in order to meet the IFR stability requirements of 14 CFR Parts 27 and 29. These systems include:
  • Aerodynamic surfaces, which impart some stability or control capability that generally is not found in the basic VFR configuration.
  • Trim systems provide a cyclic centering effect. These systems typically involve a magnetic brake/spring device and may be controlled by a four-way switch on the cyclic. This system requires “hands on” flying of the helicopter.
  • Stability Augmentation Systems (SAS) provide short-term rate damping control inputs to increase helicopter stability. Like trim systems, SAS requires “hands-on” flying.
  • Attitude Retention Systems (ATT) return the helicopter to a selected attitude after a disturbance. Changes in attitude can be accomplished usually through a four- way “beep” switch or by actuating a “force trim” switch on the cyclic, which sets the desired attitude manually. Attitude retention may be a SAS function or may be the basic “hands off” autopilot function.
  • Autopilot Systems (APs) provide for “hands off” flight along specified lateral and vertical paths. The functional modes may include heading, altitude, vertical speed, navigation tracking, and approach. APs typically have a control panel for mode selection and indication of mode status. APs may or may not be installed with an associated FD. APs typically control the helicopter about the roll and pitch axes (cyclic control) but may also include yaw axis (pedal control) and collective control servos.
  • Flight Directors (FDs) provide visual guidance to the pilot to fly selected lateral and vertical modes of operation. The visual guidance is typically provided by a “single cue,” commonly known as a “vee bar,” which provides the indicated attitude to fly and is superimposed on the attitude indicator. Other FDs may use a “two cue” presentation known as a “cross pointer system.” These two presentations only provide attitude information. A third system, known as a “three cue” system, provides information to position the collective as well as attitude (roll and pitch) cues. The collective control cue system identifies and cues the pilot what collective control inputs to use when path errors are produced or when airspeed errors exceed preset values. The three-cue system pitch command provides the required cues to control airspeed when flying an approach with vertical guidance at speeds slower than the best-rate-of-climb (BROC) speed. The pilot manipulates the helicopter’s controls to satisfy these commands, yielding the desired flightpath or may couple the autopilot to the FD to fly along the desired flightpath. Typically, FD mode control and indication are shared with the autopilot. Pilots must be aware of the mode of operation of the augmentation systems and the control logic and functions in use. For example, on an instrument landing system (ILS) approach and using the three-cue mode (lateral, vertical, and collective cues), the FD collective cue responds to glideslope deviation, while the horizontal bar cue of the “crosspointer” responds to airspeed deviations. However, the same system when operated in the two-cue mode on an ILS, the FD horizontal bar cue responds to glideslope deviations. The need to be aware of the FD mode of operation is particularly significant when operating using two pilots.

Pilots should have an established set of procedures and responsibilities for the control of FD/AP modes for the various phases of flight. Not only does a full understanding of the system modes provide for a higher degree of accuracy in control of the helicopter, it is the basis for crew identification of a faulty system.

Helicopter Flight Manual Limitations

Helicopters are certificated for IFR operations with either one or two pilots. Certain equipment is required to be installed and functional for two-pilot operations and additional equipment is required for single-pilot operation.

In addition, the Helicopter Flight Manual (HFM) defines systems and functions that are required to be in operation or engaged for IFR flight in either the single or two-pilot configurations. Often, in a two-pilot operation, this level of augmentation is less than the full capability of the installed systems. Likewise, a single-pilot operation may require a higher level of augmentation.

The HFM also identifies other specific limitations associated with IFR flight. Typically, these limitations include, but are not limited to:
  • Minimum equipment required for IFR flight (in some cases, for both single-pilot and two-pilot operations)
  • VMINI (minimum speed—IFR) [Figure 1]
  • VNEI (never exceed speed—IFR)
  • Maximum approach angle
  • Weight and center of gravity (CG) limits
  • Helicopter configuration limitations (such as door positions and external loads)
  • Helicopter system limitations (generators, inverters, etc.)
  • System testing requirements (many avionics and AFCS, AP, and FD systems incorporate a self-test feature)
  • Pilot action requirements (for example, the pilot must have hands and feet on the controls during certain operations, such as an instrument approach below certain altitudes)
Helicopter Instrument Flight Rule (IFR) Certification
Figure 1. VMINI limitations, maximum IFR approach angles and G/A mode speeds for selected IFR certified helicopters

Final approach angles/descent gradient for public approach procedures can be as high as 7.5 degrees/795 ft/NM. At 70 knots indicated airspeed (KIAS) (no wind), this equates to a descent rate of 925 fpm. With a 10-knot tailwind, the descent rate increases to 1,056 fpm. “Copter” Point-in-space (PinS) approach procedures are restricted to helicopters with a maximum VMINI of 70 KIAS and an IFR approach angle that enables them to meet the final approach angle/descent gradient. Pilots of helicopters with a VMINI of 70 KIAS may have inadequate control margins to fly an approach that is designed with the maximum allowable angle/descent gradient or minimum allowable deceleration distance from the missed approach point (MAP) to the heliport. The “Copter” PinS final approach segment is limited to 70 KIAS since turn containment and the deceleration distance from the MAP to the heliport may not be adequate at faster speeds. For some helicopters, engaging the autopilot may increase the VMINI to a speed greater than 70 KIAS, or in the “go around” (G/A) mode, require a speed faster than 70 KIAS. [Figure 1] It may be possible for these helicopters to be flown manually on the approach or on the missed approach in a mode other than the G/A mode.

Since slower IFR approach speeds enable the helicopter to fly steeper approaches and reduces the distance from the heliport that is required to decelerate the helicopter, you may want to operate your helicopter at speeds slower than its established VMINI. The provision to apply for a determination of equivalent safety for instrument flight below VMINI and the minimum helicopter requirements are specified in Advisory Circulars (AC) 27-1, Certification of Normal Category Rotorcraft and AC 29-2, Certification of Transport Category Rotorcraft. Application guidance is available from the Rotorcraft Directorate Standards Staff, ASW-110, 2601 Meacham Blvd., Fort Worth, Texas, 76137­4298, (817) 222-5111.

Performance data may not be available in the HFM for speeds other than the best rate of climb speed. To meet missed approach climb gradients, pilots may use observed performance for similar weight, altitude, temperature, and speed conditions to determine equivalent performance. When missed approaches utilizing a climbing turn are flown with an autopilot, set the heading bug on the missed approach heading, and then at the MAP, engage the indicated airspeed mode, followed immediately by applying climb power and selecting the heading mode. This is important since the autopilot roll rate and maximum bank angle in the Heading Select mode are significantly more robust than in the NAV mode. Figure 2 represents the bank angle and roll limits of the S76 used by the FAA for flight testing. It has a roll rate in the Heading Select mode of 5 degrees per second with only 1 degree per second in the NAV mode. The bank angle in the Heading Select mode is 20 degrees, with only 17 degrees in the NAV Change Over mode. Furthermore, if the Airspeed Hold mode is not selected on some autopilots when commencing the missed approach, the helicopter accelerates in level flight until the best rate of climb is attained, and only then will a climb begin.

Helicopter Instrument Flight Rule (IFR) Certification
Figure 2. Autopilot bank angle and roll rate limits for the S-76 used by the William J. Hughes Technical Center for Flight Tests

WAAS localizer performance (LP) lateral-only PinS testing conducted in 2005 by the FAA at the William J. Hughes Technical Center in New Jersey for helicopter PinS also captured the flight tracks for turning missed approaches. [Figure 3] The large flight tracks that resulted during the turning missed approach were attributed in part to operating the autopilot in the NAV mode and exceeding the 70 KIAS limit.

Helicopter Instrument Flight Rule (IFR) Certification
Figure 3. Flight tests at the William J. Hughes Technical Center point out the importance of airspeed control and using the correct technique to make a turning missed approach

Operations Specifications

A flight operated under 14 CFR Part 135 has minimums and procedures more restrictive than a flight operated under 14 CFR Part 91. These Part 135 requirements are detailed in their operations specifications (OpSpecs). Helicopter Air Ambulance (HAA) operators have even more restrictive OpSpecs. Shown in Figure 4 is an excerpt from an OpSpecs detailing the minimums for precision approaches. The inlay in Figure 4 shows the minimums for the ILS Runway 3R approach at Detroit Metro Airport. With all lighting operative, the minimums for helicopter Part 91 operations are a 200-foot ceiling, and 1,200-feet runway visual range (RVR) - one-half airplane Category A visibility but no less than 1⁄4 SM/1,200 RVR. However, as shown in the OpSpecs, the minimum visibility this Part 135 operator must adhere to is 1,600 RVR. Pilots operating under 14 CFR Part 91 are encouraged to develop their own personal OpSpecs based on their own equipment, training, and experience.

Helicopter Instrument Flight Rule (IFR) Certification
Figure 4. Operations Specifications

Minimum Equipment List (MEL)

A helicopter operating under 14 CFR Par t 135 with certain installed equipment inoperative is prohibited from taking off unless the operation is authorized in the approved MEL. The MEL provides for some equipment to be inoperative if certain conditions are met. [Figure 5]

Helicopter Instrument Flight Rule (IFR) Certification
Figure 5. Example of a Minimum Equipment List (MEL)

In many cases, a helicopter configured for single-pilot IFR may depart IFR with certain equipment inoperative provided a crew of two pilots is used. Under 14 CFR Part 91, a pilot may defer certain items without an MEL if those items are not required by the type certificate, CFRs, or airworthiness directives (ADs), and the flight can be performed safely without them. If the item is disabled, removed, or marked inoperative, a logbook entry is made.

Previous Post Next Post