Browse the Glossary

Passenger Revenue per Available Seat Mile (PRASM)

Passenger Revenue per Available Seat Mile is a  common measure of an Airline’s “unit revenue” or how much profit the Airline is making for each unit of capacity. It is calculated by dividing passenger revenue by available seat miles. Typically the measure is presented in terms of cents per mile. This measure is equivalent to the product of load factor and yield

Unmanned Aircraft (UA) / Unmanned Aircraft System (UAS)

OPAUnmanned Aircraft (UA) – FAA Definition – A device used or intended to be used for flight in the air that has no onboard pilot. The device can be any type of airplane, helicopter, airship, glider (powered or unpowered), powered-lift aircraft, or tethered aircraft without an onboard pilot. Unmanned free balloons and unmanned rockets discussed in 14 CFR part 101 are not considered UA.

Unmanned Aircraft System (UAS) – FAA Definition – An unmanned aircraft and its associated elements related to safe operation, which may include control stations, data links, support equipment, payloads, flight termination systems, and launch/recovery equipment.





Reference FAA Order 8130.34C  Airworthiness Certification of Unmanned Aircraft Systems and Optionally Piloted Aircraft

Optionally Piloted Aircraft (OPA)

OPAOptionally Piloted Aircraft (OPA) – A manned aircraft that can be flown by a remote pilot from a location not onboard the aircraft.

Reference FAA Order 8130.34C  Airworthiness Certification of Unmanned Aircraft Systems and Optionally Piloted Aircraft


Eastern Airlines Flight 401 Accident – Aviation Human Factors Case Study

 Eastern_Air_Lines L-1011 Tristar flight 401 aircraftAviation Human Factors –

Eastern Airlines Flight 401 is a classic case study of aviation human factors that I’m sure many of you are familiar with but one that deserves revisiting.

Eastern Air Lines Aircraft 310EA (seen here) a Lockheed L-1011 crashed on December 29, 1972, 18.7 miles west-northwest of Miami International Airport, Miami, Florida enroute from JFK Airport.  The aircraft was destroyed.

Aviation Errors can have Tragic Consequences

Of the 163 passengers and 13 crew-members aboard, 94 passengers and 5 crewmembers received  fatal injuries. Two survivors died later as a result of their injuries.

More Experience = More Complacency?

The Captain was Robert Albin ‘Bob’ Loft, 55 a 30 year veteren employee who had 29,000 hours and was #50 on the Pilot seniority list. First Officer Albert Stockhill  had 5800 hours and Flight Engineer (Second Officer) had over 15,000 flight hours and held commercial and Airframe & Powerplant Mechanic’s certificates.

Until the approach to MIA the flight was uneventful. The landing gear handle was placed in the “down” position during the preparation for landing, and the green light, which would have indicated to the flightcrew that the nose landing gear was fully extended and locked, failed to illuminate. The Captain executed a “Missed Approach”  and  climbed the aircraft to 2,000 feet proceeding on a westerly heading towards the Everglades.

It’s Just a Light Bulb

The first officer successfully removed the nose gear light lens assembly, but it jammed when he attempted to replace it. The captain then  instructed the Second Officer to enter the forward electronics bay located below the flight deck, to visually check the alignment of the nose gear indicator stripes thus verifying that the gear was securely down and locked. Apparently the indicator stripes were not easily verifiable because the wheel well light was not operational.

Meanwhile, the flightcrew continued their attempts to free the nose gear position light lens from its retainer, without success. The Captain and the First Officer continued to discuss the lens assembly and how it might have been reinserted incorrectly.

Seemingly Minor Maintenance Tasks Actually Matter

Shortly after 2341, the second officer raised his head into the flight Deck and stated, “I can’t see it, it’s pitch dark and I throw the little light, I get, ah, nothing. ” We can only speculate on whether the wheel well light was operational and the optical tube was clean and visible.

The flightcrew and an Eastern Air Lines maintenance specialist  Angelo Donadeo, who was occupying the forward observer seat discussed the operation of the nose wheel well light. Afterward, Angelo went into the electronics bay to assist the second officer to verify the gear was down and locked.

Tunnel Vision and Lack of Situational Awareness

The three flight crewmembers and the jumpseat occupant all became engrossed in the malfunction.

The National Transportation Safety Board determined that the probable cause of the accident was the failure of the flightcrew to monitor the flight instruments during the final 4 minutes of flight, and to detect an unexpected descent soon enough to prevent impact with the ground.

Preoccupation with a malfunction of the nose landing gear position indicating system distracted the crew’s attention from the instruments and allowed the descent to go unnoticed.

Distraction, confusion and lack of effective coordination amongst the crew led to the event.

Lessons Learned

It’s wise to reflect on the the lessons learned of how a seemingly simple procedure (replacing a light bulb and visually verifying the nose landing gear is down and locked) may have tragic consequences if done incorrectly.

It’s a Leaders responsibility to look at the whole process and monitor progress. It’s your team members responsibility to stay focussed and complete their individual responsibilities. It’s everyones  responsibility to understand  and not lose sight of the ultimate mission especially when multitasking and being stretched thin. In this case the mission was to land the aircraft safely and they failed.

Here’s a FAA training video recreating the final moments of flight 401 from the CVR transcript

A copy of the NTSB report is available here:


A review of the “Dirty Dozen” is here:

Printable copy of the Aviation Dirty Dozen

More information on Eastern airlines Flight 401 accident may be found here:


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Aircraft Approach Category

Aircraft approach category means a grouping of aircraft based on a Vref (reference landing speed), if specified, or if Vref is not specified, 1.3 Vso (stalling speed or minimum steady flight speed in the landing configuration), both at the maximum certificated landing weight. Vref, Vso, and the maximum certificated landing weight are those values as established by the certification authority. In 2002, 14 CFR  97.3 was changed to include Vref as well as 1.3 Vso to establish approach category.

An aircraft’s approach category does not change if the actual landing weight is less than the maximum certificated landing weight. The certificated approach category is permanent and independent of the changing conditions of day-to-day operations. An aircraft is certificated in only one approach category and cannot be flown to the minimums of a slower approach category, e.g., a category C aircraft cannot utilize category B minimums.

Pilots are responsible for determining if a higher approach category applies

If the requirement for a faster approach speed places the aircraft in a higher speed approach category, the minimums for the appropriate higher category must be used, e.g., emergency returns requiring overweight landing, approaches made with inoperative flaps or in icing conditions, e.g., category C aircraft may be required to apply category D minimums.

The Approach categories are as follows:

(1) Category A: Speed less than 91 knots.
(2) Category B: Speed 91 knots or more but less than 121 knots.
(3) Category C: Speed 121 knots or more but less than 141 knots.
(4) Category D: Speed 141 knots or more but less than 166 knots.
(5) Category E: Speed 166 knots or more

This definition and explanation of Aircraft Approach Category was published in a 2012 FAA SAFO

SAFO – Safety Alerts for Operators – FAA Aviation Safety Information

Aviation Safety Alert OperatorsSafety Alerts for Operators published by the FAA often contain critical aviation safety related information that the FAA has determined  that various aircraft operators need to know.

Increases Aviation Safety Communication

The SAFO is intended to fill the gap left by the discontinuation in 1994 of the ACOB (Air Carrier Operational Bulletin) and attempts to re-establish a communication channel between FAA Flight Standards and operators. This channel permits a policy making division such as AFS-200 (Air Transportation Division) to convey important safety information concurrently to FAA field offices and to operators in a timely manner, without undue delays caused by bureaucracy between government offices.

SAFO’s are not regulatory required but may contain information and actions that are recommended to be taken voluntarily by the respective types of operators identified in each SAFO. The content of these alerts is intended to be valuable to air carriers in meeting their statutory duty to provide service with the highest possible degree of Aviation safety in the public interest.

Safety Alert Audience

  • Part 121 and 135 Air Carrier certificate holders
  • Managers of  Fractional Aircraft ownership programs
  • Aviation Training Center Managers
  • Directors of Maintenance ( D.O.M.’s) of part 121 and part 135 Air carriers
  • Accountable Managers at part 145 repair stations.

FAA Safety Alert Distribution

SAFOs are maintained by the FAA Flight Standards Service Air Transportation Division and are available for download on the FAA website arranged in categories and listed in chronological order.

In addition to being published on the FAA website, the FSS notifies its Region Managers, Aircraft Evaluation Group, Certificate Management Offices, Flight Standards District Offices and International Field Offices of new SAFOS.

Recently Issued Aviation Safety Alerts for Operators

 FAA promoting Manual Flight Operations

Chaffing, Arcing, and Burning Damage to Boeing 737-100/200/300/400/500 Series Aircraft Flight Deck Overhead Wiring-Ducting

Title 14 of the Code of Federal Regulations (14 CFR) Part 121Air Carriers Transporting Heavy Vehicle Special Cargo Loads


A “power back” procedure is where reverse engine thrust is utilized to “back up” an aircraft in contrast to a “push back” where a vehicle is used to move the aircraft off of a gate. Utilizing a ground “marshaller” who directs the pilots, the aircraft thrust reversers are deployed and the engine throttles are advanced from idle.. As reverse thrust is increased, backward motion increases.

Since there is no way for the pilots to see what is behind them, the pilots are completely dependent on ground personnel to keep them from running into objects.

Using this procedure has a few serious associated hazards that generally limit its use generally to aircraft with tail mounted engines at airports which allow the procedure and at remote locations used as a last resort where suitable ground equipment is not available to “push back” the aircraft.

One of the major hazards is the risk of FOD or Foreign Object Damage (debris)  to the engines and ground personnel. Prior to the late 1980’s the most widely produced passenger jet was the Boeing 727 followed by the DC-9 / MD80 series. Both of these aircraft have tail mounted engines that are elevated from the ground and less susceptible to ingesting objects littering the ramp areas. Since the 1990’s, the most widely produced aircraft have been the 737 series with 7467 produced (as of January 2013) followed by the Airbus 320 family of which 5635 have been produced as of June 2013 and these aircraft both have wing mounted engines that are positioned very close to the ground. Both the risk of engine ingestion with possible engine damage and flying objects propelled forward towards the terminal and ground personnel are real threats. Airport gate areas are often littered with items that fall off of passengers luggage such as zipper pulls, wheels, pull rings etc and even though most ramp personnel are instructed to monitor and pick up these items the “FOD walks” are generally done when the gate area is empty since most engine starts occur during or after the aircraft is pushed back and only run at idle.

I also speculate that since the tail mounted engines are much closer to the aircraft centerline than the wing mounted engines that the tail mounted engine aircraft are easier to control especially if there is any difference in the engines thrust. I speculate because I have never “powered back” a 737 or Airbus.

The risk of damage by collision with other aircraft or airport equipment is much higher when performing a power back primarily because the marshaller is standing in front of the aircraft using hand signals and maintaining visual contact with the flight deck and is unable to fully observe the area aft of the aircraft.

Many Airports have restrictions on aircraft using above idle power settings at the gate due to safety concerns of Jet Blast and noise. For example BWI (Baltimore, Washington International Thurgood Marshall) states in their tenant restrictions that ” Power backs exceeding a noise level of 120 dB, measured on the Airport terminal roof above the gate in question, shall not be authorized” and “Reverse-thrust power-back operations are prohibited at BWI Marshall between 2300 and 0700 local hours daily.”

Wear and tear on million dollar engines: The heat and stress on engines when using reverse thrust is higher. Also, the airflow is often disrupted increasing the chance for compressor stalls which require a Maintenance defect entry and usually a borescope inspection of the engine for internal damage to the compressor and HPT.

So, when airlines evaluate the risks outlined above they usually come to the conclusion that there is not enough economic advantage to be gained by allowing power backs. If they are allowed, they’re  only permitted under the conditions outlined in the Air Carrier Flight and Operations manual and with prior approval from the airport authorities.

Today with the significantly increased focus on awareness of both safety and compliance in the Airline Industry I don’t think we’ll see very many “power backs” going forward. That’s a good thing in my opinion.  In my world, the days of doing whatever it took to get the aircraft off the gate regardless of what the manual said to do are long gone.

Have any of you had the opportunity to power back either in the left seat or direct one from the ground? If so I’d love to hear your experiences. What Airports are powerbacks still utilized?




Line Maintenance



Line Maintenance






Line Maintenance generally refers to minor, unscheduled or scheduled maintenance carried out on aircraft that includes:

  1. Any unscheduled maintenance resulting from unforeseen events
  2. Scheduled checks that contain servicing and/or inspections that do not require specialized training, equipment, or facilities.In service; and that is preparing for its first flight in service
  3. Maintenance performed on aircraft after a period of being out of service (such as aircraft in storage)
  4. Maintenance on en route aircraft that are stopped before their next flight including Servicing or repair between successive flights
  5. Preparing and readying an aircraft for flight during a period of service
  6. Maintenance activities being performed  to ensure that the aircraft is airworthy and fit for flight.


The three common areas where Aircraft Maintenance is performed are:

Line or  Flight Line Maintenance

Occurs at or near the gate or terminal (Tarmac), launch area, ready area, hardstand or alert area. The level of dis-assembly is usually limited to what can be reassembled and restored within a period of less than a shift or two. A limiting factor is usually the level of Ground Support Equipment required such as Electrical Carts, Hydraulic Mules, work stands and lifts. Various safety regulations limit the amount of work that may be practically performed outside of a hangar facility

Hangar Maintenance or Intermediate Maintenance

Any level of Maintenance from Servicing to Overhaul may be performed inside a hangar.  Typically most organizations utilize a facility where either the complexity, length of repair, number of staff, support equipment, tools and parts required dictate a covered, dedicated facility. Most repairs conducted at this level are usually “on-wing” or simple parts replacement.

Heavy Maintenance, MRO or Overhaul facility

The highest level of Maintenance with capabilities of disassembling  Inspecting, repairing, refurbishment, Overhaul and restoration. Often the capabilities of the facility will include a repair station certificated under part 145 where off wing  repair and overhaul of individual components may also be performed.

In fact by virtue of being certificated under part 145 requires the entity to maintain a controlled facility where the work is being performed and when working outside  they must have facilities that are acceptable to the FAA which is essentially advance approval. See 14 CFR 145.103 (c) below


145.103   Housing and facilities requirements.

(a) Each certificated repair station must provide—

(1) Housing for the facilities, equipment, materials, and personnel consistent with its ratings.

(2) Facilities for properly performing the maintenance, preventive maintenance, or alterations of articles or the specialized services for which it is rated. Facilities must include the following:

(i) Sufficient work space and areas for the proper segregation and protection of articles during all maintenance, preventive maintenance, or alterations;

(ii) Segregated work areas enabling environmentally hazardous or sensitive operations such as painting, cleaning, welding, avionics work, electronic work, and machining to be done properly and in a manner that does not adversely affect other maintenance or alteration articles or activities;

(iii) Suitable racks, hoists, trays, stands, and other segregation means for the storage and protection of all articles undergoing maintenance, preventive maintenance, or alterations;

(iv) Space sufficient to segregate articles and materials stocked for installation from those articles undergoing maintenance, preventive maintenance, or alterations; and

(v) Ventilation, lighting, and control of temperature, humidity, and other climatic conditions sufficient to ensure personnel perform maintenance, preventive maintenance, or alterations to the standards required by this part.

(b) A certificated repair station with an airframe rating must provide suitable permanent housing to enclose the largest type and model of aircraft listed on its operations specifications.

(c) A certificated repair station may perform maintenance, preventive maintenance, or alterations on articles outside of its housing if it provides suitable facilities that are acceptable to the FAA and meet the requirements of § 145.103(a) so that the work can be done in accordance with the requirements of part 43 of this chapter.


Depot Level Maintenance (US Air Force Definition)

Depot-Level Maintenance. The level of maintenance consisting of those on and off-equipment tasks performed using
highly specialized skills, sophisticated shop equipment, or special facilities of an ALC, contractor facility, or, by field teams
at an operating location. Maintenance performed at a depot also includes those organizational- and intermediate-level tasks
required to prepare for depot maintenance, and, if negotiated between the depot and the operating command, scheduled
field-level inspections, preventative maintenance or TCTOs which come due while equipment is at the ALC for PDM.

System Safety Assessment

Safety assessments are a primary means of compliance for systems that are critical to safe flight and operation. Safety assessments proceed in a stepwise, data-driven manner. Functional hazard assessments are performed to identify the failure conditions associated with each airplane function, and system functional hazard analyses are performed for system-level functions.

The bottom-up verification starts with a safety analysis of the components of a system to ensure that single failures do not result in significant effects.

Combinations of failures are then analyzed to develop the probability of a failure and checked to ensure that the probability is commensurate with the criticality of the failure condition. Thus, the final definition and characterization of a safety-critical system is verified by the result of the analyses conducted during a safety assessment.

Class I Navigation as described in Part 121 OpSpecs and VFR Class I Navigation

Class I navigation is any en route flight operation or portion of an operation that is conducted entirely within the designated Operational Service Volumes (or ICAO equivalents) of ICAO standard airway navigation facilities (VOR, VOR/DME, NDB). Class I navigation also includes en route flight operations over routes designated with an “MEA GAP” (or ICAO equivalent). En route flight operations conducted within these areas are defined as “Class I navigation” operations irrespective of the navigation means used. Class I navigation includes operations within these areas using pilotage or any other means of navigation which does not rely on the use of VOR, VOR/DME, or NDB.

VFR CLASS I NAVIGATION. Visual flight rules (VFR) Class I navigation is any Class I navigation operation conducted under VFR in visual meteorological conditions (VMC).

The primary objectives of VFR Class I navigation are as follows:
·       Arrive at the intended destination with sufficient fuel remaining to safely complete a landing

·       Operate with sufficient visual references to reliably “see and avoid” all obstacles along the actual routes of flight

·       Operate with sufficient visibility to safely “see and avoid” all other aircraft

·       Navigate with sufficient precision to avoid special area of operation areas and positive air traffic control (ATC) are
or to comply with the special requirements of those areas

·       Protect persons and property on the ground, which is an important factor in route selection and route approval especiall for those aircraft that have inadequate performance capability with an engine inoperative

Conduct of VFR Flight. In the conduct of VFR flight, the prevention of collisions (safe separation from other aircraft) is solely the responsibility of the pilot-in-command (PIC) to see and avoid. However, there are regulatory requirements for use of navigation systems such as VOR for VFR operations in oceanic or desolated land areas or for night VFR and VFR over the top operations. These regulatory requirements are related to locating the intended destination, avoiding obstacles along the actual route of flight, and the protection of persons and property on the ground.
TYPES OF VFR CLASS I NAVIGATION. These are two types of VFR Class I navigation. They are referred to as “pilotage ” and “station referenced.”
Pilotage. One of the primary means of conducting VFR Class I navigation is by pilotage. Pilotage is defined in Title 14 of the Code of Federal Regulations (14 CFR) part 1 as “navigation by visual reference to landmarks.”
Pilotage is an appropriate means of navigation only in those areas and/or situations where the flight conditions (ceiling and visibility) are sufficient to consistently identify prominent landmarks and to “see and avoid” obstacles and other aircraft. Examples of prominent landmarks include villages, rivers, roads, valleys, ridges, transmission lines, and in some cases, lighted objects at night.
Pilotage is not an appropriate means of VFR Class I navigation in areas or situations where prominent landmarks or lighted objects do not exist or where these visual references are widely separated. For example, desolate areas without prominent and permanent features, such as deserts, the Tar Pits in Canada, huge forests, certain Arctic areas, or large bodies of water (such as parts of the Great Lakes and the Gulf of Mexico), are areas where pilotage is not an appropriate means of navigation.

Station-Referenced. In situations where pilotage is not appropriate, it is necessary to use other means of conducting VFR Class I navigation to locate the intended destination, avoid obstacles, and protect persons and property on the ground. This is accomplished by using electronic station-referenced (nonvisual) navigational aids (NAVAID), such as VOR, DME, NDB, or LORAN-C, and GNSS.
Conventional ground-based NAVAIDs (VOR, DME, NDB ) can be used to fly published routes. In this case, obstacle avoidance is provided if the operation is conducted at or above the published minimum en route IFR altitude minimum en route altitude (MEA) or (if appropriate) the minimum obstruction clearance altitude (MOCA).
Area navigation systems can be used to conduct VFR Class I navigation. Most area navigation systems are station-referenced systems; however, an inertial navigation system (INS) is self-contained and the Global Navigation Satellite System (GNSS) is space based. Although these systems are referenced to specific navigation stations (VOR, VOR/DME, and LORAN-C), area navigation systems permit point-to-point navigation and are not limited to routes from one ground station to the next. Since the VFR navigation performance requirements are not as demanding as IFR requirements, operators can use area navigation systems for VFR that are not certificated for IFR en route operations. However, certain systems, such as LORAN-C and GPS, must be certified as airworthy for VFR and installed in accordance with approved documentation.VFR CLASS I NAVIGATION APPROVALS.
Determining Degree of Accuracy. In determining the degree of accuracy required for pilotage and station-referenced VFR Class I navigation, an inspector must consider the minimum flight conditions necessary for safe operations. If it is determined that flight conditions better than basic VFR weather minima are required for safe operations, the specific flight conditions (e.g., ceiling visibility) must be specified in the operations specifications (OpSpecs) for the pertinent area or route. When making this determination for station referenced Class I navigation, consideration should be given to the additional accuracy provided by the electronic navigation equipment. In addition, station referenced navigation requires that the navigational equipment used is airworthy for VFR operations within the proposed area of operation and installed in accordance with approved data. The operator must provide written evidence of the airworthiness approval for the required equipment.

When a minimum flight condition for either pilotage or station referenced Class I navigation is specified in OpSpecs, it must provide for the following criteria:
·    Meets regulatory requirements for the operation

·    Meets the standard practices in this handbook

·    Meets the requirements of part B of the OpSpecs

·    Provides accepted, safe operating practice

·    Permits “see and avoid”

·    Permits the identification and avoidance of obstacles

·    Ensures adequate protection of persons and property on the ground

·    Permits reliable identification of prominent landmarks or lighted objects at night

·    Permits reliable navigation to the intended destination
Pilotage and Station-Referenced Approvals. Pilotage and station-referenced approvals are granted by issuance or amendments to OpSpecs . The areas of operation authorized for pilotage or station-referenced Class I VFR navigation, along with any required minimum flight condition, must be specified in the OpSpecs.

Area Navigation Systems.

·    VOR-DME

·    DME-DME

·    LORAN-C

·    GPS

IFR Class I navigation is any Class I navigation operation conducted under IFR. The following are the primary objectives of IFR Class I navigation:
·    Navigating with sufficient precision to permit ATC to safely separate IFR aircraft.

·    Arriving at the intended destination with adequate fuel remaining to safely complete a landing.

·    Avoiding all obstacles along the actual route of flight.

·    Providing adequate protection for persons and property on the ground, especially for those aircraft with inadequate performance capability with an inoperative engine(s).

·    Meeting the requirements of part B of OpSpecs.
Safe Separation of Aircraft. Since the safe separation of aircraft under IFR in controlled airspace is dependent on the aircraft’s navigational performance, an inspector must determine that the navigational equipment and the navigation procedures and techniques used by the operator ensure that the operation will be conducted with the precision necessary to meet the objectives listed in the previous subparagraph. Inspectors must consider the following when approving IFR Class I navigation:
·    Situations when the means of navigation is other than VOR or VOR/DME will normally require a case‑by-case evaluation.

·    In all cases, the means of navigation must enable navigation to the degree of accuracy required for the control of air traffic.

·    IFR Class I navigation is only conducted within the operational service Volume of standard International Civil Aviation Organization (ICAO) NAVAIDs.

TYPES OF IFR CLASS I NAVIGATION. There are two generic types of IFR Class I navigation:
·    Navigation by direct reference to ICAO standard NAVAIDs

·    Navigation by use of RNAV systems
ICAO Standard NAVAIDs. The primary means of conducting IFR Class I navigation has historically been station-referenced to ICAO standard ground-based NAVAIDs (VOR, VOR/DME, NDB). The route structure and the ATC separation standards in most countries are based on the use of these ground-based NAVAIDs. When operating within the operational service volumes of these ground-based NAVAIDs, these standard systems may be used to satisfy the objectives of IFR Class I navigation. However, with the implementation of GPS, ICAO now includes GPS as an additional standard NAVAID. Two subtypes of IFR Class I navigation can be conducted using ICAO standard NAVAIDs: ground-based or space-based. These subtypes are navigation on published IFR routes and point-to-point IFR navigation
Within the United States and Canada, standard NAVAIDs may be used to conduct Class I navigation when flying any published IFR route or procedure, provided these operations are conducted at or above the published minimum IFR altitudes. The following are examples of published IFR routes:
·    Victor airways

·    Colored airways

·    Jet/high level routes

·    Standard Instrument Departures (SID)

·    Standard Terminal Arrivals (STAR)

·    Instrument departures.

NOTE: This also includes those cases where the route is published with a “gap” in signal coverage.

In many foreign countries and in oceanic/remote areas, the situation is more complex. The determination of whether Class I navigation is appropriate must be based on ICAO standards or their equivalence to U.S. standards. In general, most published VOR and VOR/ DME routes (airways) are equivalent to U.S. standards and IFR Class I navigation can be conducted over these routes using standard VOR, VOR/DME equipment. In many areas outside the United States and Canada, some of the published routes are based on NDBs . Any published route must be evaluated to determine whether the route involves Class I or Class II navigation, or both. For example, if the entire portion of a route based on NDB is determined to be Class I navigation, NDB equipment is usually sufficient to conduct airway navigation over that route when flying at or above the specified minimum IFR altitude. Point-to-point IFR Class I navigation based on NDBs generally requires a case-by-case evaluation to ensure the operation will be conducted in accordance with ICAO or U.S. standards. The fact that the route is approved by the ICAO contracting State does not automatically mean that the route meets these safety criteria.

  IFR Class I navigation can be conducted over unpublished point to point routes (off airways), provided all of the following conditions are met:

·    Positive course guidance is available from standard ICAO NAVAIDs.

·    The routes are within the operational service Volume of these NAVAIDs.

·    The operation is conducted at or above the IFR minimum altitude published or approved for that route by the ICAO contracting state having jurisdiction over that airspace.

·    The required airborne, ground-based and/or space-based navigational facilities are available and operational to enable navigation to the degree of accuracy required for the control of air traffic.
Area Navigation Systems. Appropriate area navigation systems can be used to conduct IFR Class I navigation. Any area navigation system used for IFR flight must provide present position information and navigation guidance to maintain the assigned track and arrive at the designated waypoints. Area navigation may be based on the following:
·    VOR and DME-source-referenced

·    LORAN-C, GNSS earth-referenced in accordance with WGS84 or equivalent

·    Self-contained in the aircraft (INS, IRS).

IFR Class I navigation can be conducted with IFR-approved area navigation systems suitable for the area of operations. Area navigation systems must be evaluated to ensure that the system and the operator are capable of navigating to the degree of accuracy required for control of air traffic within the proposed area of operation<
In U.S. Class A airspace (18,000 feet mean sea level (MSL) to Flight Level (FL) 600), IFR Class I navigation can be conducted with suitable area navigation systems that are not approved for IFR flight in areas where domestic ATS procedures are applied. In the U.S. Class A airspace, additional safety is provided by ATC radar. This independent surveillance method and the procedures specified for this type of operation provides an equivalent level of safety and permits safe separation of aircraft. RNAV operations can be authorized provided the following conditions are met:
·    The flightcrew is properly trained for the equipment and special procedures to be used.

·    Each flight operation is authorized by the appropriate ATC facility.

·    The entire portion of the intended route of flight using the area navigation system will be in the U.S. Class A airspace and under positive radar control.

·    Contingency procedures are established so that the flight can immediately return to and use airways facilities at any point in the flight.

·    The airborne navigational equipment (VOR, DME, ADF) required to navigate in Class A airspace is installed and operational.
IFR CLASS I NAVIGATION APPROVALS. General direction and guidance of air navigation approvals are in section 2. Specific direction and guidance for approving IFR Class I navigation is discussed in the following subparagraphs.
The navigational equipment and the operational procedures/techniques used permit reliable IFR Class I navigation to the degree of accuracy required for the control of air traffic. The degree of accuracy required for any IFR Class I navigation operation must provide for the following criteria:
·    Meets regulatory requirements for IFR airways navigation

·    Meets the standard practices in this order

·    Meets the requirements of part B of the OpSpecs

·    Provides accepted, safe operating practices

·    Permits the safe separation of aircraft

·    Ensures obstacle avoidance along the route of flight

·    Ensures adequate protection for persons and property on the ground

·    Permits reliable navigation to the intended destination and any necessary alternate or diversionary airports
The required navigational equipment must be certified for IFR flight and installed in accordance with approved data. The operator must provide written evidence of the airworthiness approval for the required equipment. The operator must also provide written evidence that shows that any RNAV system used for IFR Class I navigation meets the performance criteria for the proposed area of operation. If, for example, the proposed area of operation includes areas of magnetic unreliability (AMU), the navigation equipment must be approved for IFR operations in that environment.
The operator’s manuals, training programs, minimum equipment lists (MELs), and company policies and practices must adequately address the proposed IFR Class I navigation operation and the equipment to be used considering the following factors:
·    Terrain characteristics

·    The operator’s experience with other aircraft and navigation systems in the area of proposed operation

·    The operator’s experience with the same aircraft and navigation in similar areas of operation

·    The need to adequately protect persons or property on the ground

·    Operations in special areas of operation, including AMU

·    Use of special means of navigation

·    Use of special navigation techniques