Ops Reference Guide
General reference material for flight operations. Always verify against the current AIP, company OM-A, and applicable regulations for your specific operation — rules vary by state and can change.
Airspace Classes (ICAO Annex 11)
ICAO defines seven airspace classes, A through G, distinguished by whether IFR and VFR flight is permitted, what ATC service is provided, and what separation is guaranteed. Not every state uses every class — the US, for example, does not use Class F.
| Class | IFR/VFR | ATC Clearance | Separation Provided | Typical Use |
|---|---|---|---|---|
| A | IFR only | Required | All aircraft separated | High-altitude enroute (e.g. above FL180/FL195) |
| B | IFR & VFR | Required (both) | All aircraft separated | Major terminal areas |
| C | IFR & VFR | Required (both) | IFR from IFR & VFR; VFR from IFR | Busy terminal areas / some enroute |
| D | IFR & VFR | Required (both) | IFR from IFR only | Smaller controlled airports |
| E | IFR & VFR | IFR only | IFR from IFR only | Controlled enroute airspace |
| F | IFR & VFR | Not required | Advisory service only | Uncommon; being phased out in many states |
| G | IFR & VFR | Not required | None — see and avoid | Uncontrolled airspace |
Rule of thumb: as you move from A toward G, the level of ATC involvement and separation guarantee decreases. Class A is the most restrictive; Class G is uncontrolled.
The Semicircular Flight Level Rule
Under ICAO Annex 2, cruising level is tied to the magnetic track (true track in designated areas near the poles or where magnetic variation is unreliable) to keep opposite-direction traffic vertically separated. Below RVSM airspace, levels are spaced at 1,000 ft intervals; within RVSM airspace (typically FL290–FL410) at 1,000 ft intervals as well, once 1,000 ft vertical separation minima were extended upward.
| Magnetic Track | Non-RVSM Odd/Even | RVSM (FL290–FL410) |
|---|---|---|
| 000°–179° | Odd thousands: FL170, 190, 210… | Odd hundreds: FL290, 330, 370, 410 |
| 180°–359° | Even thousands: FL180, 200, 220… | Even hundreds: FL310, 350, 390 |
Some states (historically the former Soviet bloc) apply a table based on true track with quadrantal-style bands instead of the standard hemispheric split — always confirm against the relevant AIP before flight planning through unfamiliar airspace.
ETOPS / EDTO
ETOPS (Extended-range Twin-engine Operational Performance Standards) — now formally EDTO (Extended Diversion Time Operations) under ICAO Annex 6, covering twins, three- and four-engine aircraft alike — governs how far a route can take an aircraft from an adequate diversion airport.
The core number is the diversion time flown at the approved one-engine-inoperative (OEI) cruise speed, in still air, to the nearest adequate airport. Common approval thresholds:
| Rating | Max Diversion Time | Typical Use |
|---|---|---|
| ETOPS-120 | 120 min | Entry-level twin-engine long-haul approval |
| ETOPS-138 | 138 min | Extension used on some North Atlantic/Pacific routes |
| ETOPS-180 | 180 min | Most transoceanic twin-engine operations |
| ETOPS-240 | 240 min | Select city pairs, higher-reliability type/operator combos |
| ETOPS-330/370 | 330–370 min | Ultra-long-haul twins (e.g. Southern Pacific, polar) on approved types |
EDTO planning also drives fuel requirements (critical fuel scenario), MEL restrictions, crew and cabin equipment (extended supplemental oxygen, extra fire suppression), and designation of enroute alternate airports that must remain above defined weather minima throughout the flight-planning window.
Polar Route Operations
Flights routing over the Arctic (typically above 78°N) or through remote polar airspace bring a distinct set of operational requirements beyond standard EDTO planning:
- Communications: HF and satellite communications (SATCOM) are required where VHF/CPDLC coverage doesn't reach — many polar tracks have defined comms-out procedures.
- Navigation: dual independent long-range navigation systems (INS/GPS) are typically required; near the poles, true track references replace magnetic ones since magnetic variation becomes unreliable close to the magnetic pole.
- Fuel & cold-weather altimetry: operators must account for fuel freeze points at very low outside air temperatures, and apply cold-temperature altimeter corrections at reporting points along the route.
- Diversion planning: similar in principle to EDTO but often with sparser, more remote alternates — some polar operations carry "polar suitable airport" requirements with limited services, requiring extra survival equipment for passengers and crew.
- Space weather: solar radiation events can degrade HF communications and increase radiation dose at high latitudes and altitudes; operators typically monitor space weather advisories and have procedures to descend or reroute if a significant event is forecast or in progress.