1. longitudinal separations based on time / distance

2. Need for Separation
Whether aircraft actually need separating depends upon the class of airspace in which the aircraft are flying,
The flight rules under which the pilot is operating the aircraft

3. Longitudinal separation
If two aircraft not laterally separated, tracks within 45 degrees of each other (or the reciprocal), then they are said to be some form of longitudinal separation existing
Longitudinal separation be based upon time or distance as measure by DME.
Rule is the 15 minute rule:
No two aircraft following the same route must come within 15 minutes flying time of each other.
In areas with good nevoid cover this reduces to 10 minutes; if the preceding aircraft is faster than the following one then this can be reduced further depending of the difference in speed.
Aircraft whose tracks bisect at more than 45 degrees said to be crossing,.

4. RVSM Airspace
Airspace between FL 290 and FL 410, both inclusive, in the following Flight Information Regions (FIRs)/Upper Flight Information Regions (UIRs)
RVSM from FL330 to FL410 in the Kolkata, Chennai Delhi and Mumbai FIRs, to allow domestic traffic that were not RVSM-approved to continue to operate at FL310 and below.

5. Performance Based Navigation
RNAV -Area Navigation-Standard Instrument Departures (Sides)/Standard Terminal Arrivals (Stars); Routes, Global Positioning System (GPS) MinimumEnroute Altitudes (MEAs; Continuous Descent Arrival CDA)/Optimized ProfileDescent (OPD)
RNP Required Navigation Performance-Special Aircraft and Aircrew Authorization Required(SAAAR) [will be changing to AR – Authorization Required]

6. Non Radar Separation Same Course & Crossing Course
In areas of insufficient radar coverage, ‘manual' separation rules apply for airport departures/arrivals and enroute flights.
DME separation example: with slower speed aircraft behind, use pilot reported positions from DME (Distance Measurement Equipment) readings

7. Minimum for Same Course Separation

8. Minima for Crossing Course Separation

9. Enroute Radar Air separation
Flights must be separated from other sectors and Special Use Airspace.
En route Stage A/DARC or Stage A/EDARC:
(a) Below Flight Level 600− 2 1/2 miles.
(b) Flight Level 600 and above− 5 miles.
• Military Operating Areas
• Prohibited Area
• Restricted Area
• Warning Area