Instrument Flight Rules
Flight under Visual Flight Rules (VFR) is very restricted. For this reason Instrument Flight Rules (IFR) have been established, to allow a properly trained pilot to safely conduct a flight through restricted visibility and cloud. Instrument flight rules (IFR) are a set of regulations that dictate how aircraft are to be operated when the pilot is unable to navigate using visual references under visual flight rules. In order for the aircraft to be flown in instrument meteorological conditions (IMC), it must be fitted with the necessary instrumentation and certified by the regulatory authority. In addition to this, the pilot must hold an instrument rating. Before an aircraft in able to fly under IFR, the pilot is required to submit an IFR flight plan to the air traffic control. This allows for aircraft to be separated in controlled airspace, and for traffic information to be provided to aircraft operating in uncontrolled airspace in order that the pilot is able to separate themselves sufficiently.
The instrument panel of an IFR aircraft can be broken down into three basic sections. The first consists of the “basic six” instruments which are used to indicate the orientation of the aircraft in space (i.e. straight and level, in a turn, climbing or descending). These are:
- An airspeed indicator
- An artificial horizon
- An altitude indicator
- Turn and slip indicator
- Directional gyro/horizontal situation indicator
- Rate of climb/descent indicator
Secondly, the navigation instruments allow the pilot to navigate the aircraft between navigation aids and carry out instrument approaches to land. This suite will generally consist of two or more of the following:
- CDI (Course deviation indicator which using signals from a VHF omnidirectional radio range [VOR] beacon or instrument landing system [ILS] equipment)
- ADF (Automatic direction finder which navigates using non-directional beacons [NDB])
Finally, engine instrumentation is required which allows the pilot to monitor the condition of the machinary as well as set the desried power settings for the phase of flight. Depending on the type of engine, this could consist of any of the following:
- Engine RPM
- Manifold Pressure
- Torque Setting
- N1 (low pressure turbine) speed
- Engine oil pressure/temperature
- Exhaust gas temperature
- Fuel flow
- Fuel tank quantity
Flight Plan And ATC Clearance
Rule 35 applies to IFR flights within controlled airspace. For a flight to comply with IFR, before any flight in this condition, a flight plan must be filed either in the air or on the ground. An IFR clearance must be obtained and flight to be carried out specifically to that clearance. A pilot flying inside controlled airspace must fly the instructions given by the controller. IFR can be cancelled at any time providing the flight can be conducted under VFR conditions.
Some of the rules
Minimum Height Rule. Inside and outside controlled airspace and aircraft flying IFR must not fly below altitude 1,000ft above the nearest obstacle within 5nm, unless stated by ATC. When flying above 3,000ft or above the appropriate transition altitude, the Quadrantal Rule should be adopted.
Taking-Off and Landing under IFR
When an aircraft takes-off on an instrument flight plan, there are several methods for the aircraft to safely reach their desired track whilst maintaining clearance from the surrounding terrain. However, every aerodrome has a specified minimum cloud ceiling and visibility requirement before aircraft can get airborne. Some departure procedures are:
- Visual departure = The pilot must maintain terrain clearance visually until above the route minimum safe altitude (MSA).
- Turn on track = The aircraft can turn onto track provided the aircraft meets the minimum climb gradient requirements.
- Radar vetors = Air traffic control provides vectors that the pilot must follow to avoid terrain.
- Standard Instrument Departures (SID) = Procedures that ensure terrain clearance and are published for each IFR aerodrome.
Once the aircraft reaches its intended destination, it must make an approach to land. Should the weather conditions permit, the aircraft may carry out a visual approach (maintain terrain clearance visually) however, if this is not the case and instrument approach will need to be carried out. Instrument approaches are either based on ground based aids (VOR, NDB or ILS) or GNSS signals (known as RNAV approaches in New Zealand).
Each of these approaches will have a specified minimum descent altitude (MDA). The pilot is not permitted to descent below this height unless the visibility requirement is met and the pilot has one of the required visual references in sight for the runway of intended landing. These references include, but are not limited to the runway threshold, runway lights or the visual approach slope indicators.
Aircraft operating under IFR use airways to navigate between locations. These airways are generally defined by ground based aids (VOR or NDB) but recent developments in performance based navigation (PBN) means many aircraft now navigate using such systems that also reply on GNSS signals for position information.
Airways charts depict the airways across a nations airways system. On these charts each airways will have a specified magnetic track that should be followed, a distance and a minimum safe altitude that ensures terrain clearance. Additional information such as minimum en-route altitude (MEA), minimum reception altitudes (MRA), and route operating limits (ROL) may also be promulgated.
Fuel Requirements Under IFR
Aircraft operating under IFR are required to carry the following minimum fuel.
If an alternate is required: Fuel for flight from the take-off aerodrome, to the destination (carry out an instrument approach if necessary) and thence fly to the alternate aerodrome plus 45 minutes (30 minutes for turbine aircraft) holding fuel.
If no alternate is required Fuel for flight from the take-off aerodrome to the destination (carry out an instrument appraoch if necessary) plus 45 minutes (30 minutes for turbine aircraft) holding fuel.
Alternate Aerodrome Requirements
When operating under IFR, the pilot is required to study the forecast weather at the destination. If this is below the minima set out in the rules, an alternate aerodrome must be selected. The forecast weather at this alternate should be better than the specified minima for that aerodrome so that, in the event that a diversion is necessary, there is a reasonable chance that landing is assured.
In New Zealand, the IFR alternate minima is specified in Part 91.
The pilot in command is required to nominate an alternate unless:
- The aerodrome of intended landing has a published instrument approach and:
- At the time of submitting the flight plan, the forecast weather at the aerodrome of intended landing indicates that, one hour either side of the estimated time of arrival (ETA), the weather will:
- Have a ceiling at least 1000 feet above the approach minima published
- Have visibility greater than 5000m or 2000m greater than the minimum visibility specified for the instrument approach.