Do you need an ATC clearance to use SLOP in oceanic airspace?

No clearance is required. SLOP offsets are crew-initiated and do not require any coordination with or notification to ATC. The crew simply programs the offset into the FMS and flies it. Position reports continue to be made as if flying the assigned centerline.

Can you offset to the left under Strategic Lateral Offset Procedures?

No. SLOP only authorizes offsets to the right of centerline — either one or two nautical miles right. Left offsets are never permitted under any circumstances. The right-only rule ensures a consistent, predictable lateral displacement across all SLOP-participating aircraft.

When must a SLOP offset be removed during an oceanic flight?

The offset must be removed before the aircraft transitions back into radar-controlled domestic airspace, where ATC will expect the aircraft to be on its assigned route centerline when radar identification is established. Crews should also verify the offset has not been inadvertently canceled by FMS route changes during the oceanic portion of the flight, since this behavior can vary by aircraft and FMS type.

Strategic Lateral Offset Procedures (SLOP) in Oceanic Airspace

Oceanic airspace presents a unique set of challenges that simply do not exist in the radar-controlled domestic environment. Once an aircraft departs radar coverage and enters the oceanic environment, controllers rely on procedural separation — fixed altitude, speed, and track assignments — rather than real-time radar vectoring. This creates a situation where aircraft may be operating on the same or adjacent tracks with limited ability for controllers to detect deviations. Strategic Lateral Offset Procedures (SLOP) were developed specifically to address the residual collision risk and wake turbulence exposure that remain even after standard procedural separation is applied.

SLOP is authorized and encouraged in oceanic and remote continental airspace where ATC radar surveillance is not available. Rather than requiring every aircraft to fly precisely on the centerline of its assigned track, SLOP permits flight crews to fly the centerline or offset laterally — to the right only — by either one or two nautical miles. This seemingly simple adjustment has a measurable impact on safety in a collision risk environment where target levels of safety are typically expressed on the order of 5×10−9 fatal accidents per flight hour per dimension (lateral, vertical, and longitudinal).

How SLOP Works

Under SLOP, a flight crew may elect to fly at the assigned track centerline, one nautical mile right of centerline, or two nautical miles right of centerline. No left offsets are ever authorized. The offset is applied without an ATC clearance — the crew makes the decision independently and does not need to notify ATC or make a position report reflecting the offset. The assigned route, altitude, Mach number, and all other ATC clearance elements remain unchanged; SLOP affects only the lateral path within the cleared route block.

The procedure is intended to be random in its application across the fleet. If every crew chose the same offset position, the safety benefit would be lost. By having some aircraft on centerline, some at one NM right, and some at two NM right, the probability that two aircraft on the same track are at the same lateral position simultaneously is significantly reduced. This randomization is fundamental to how SLOP achieves its risk-reduction benefit.

SLOP also addresses wake turbulence. At high altitudes in oceanic airspace, aircraft may be separated by as little as 1,000 feet vertically on the same track. A heavy or super aircraft cruising ahead on centerline generates wake vortices that descend and can persist for several minutes in stable air. An aircraft arriving at that same point on centerline — even many miles behind — may encounter those vortices. By offsetting right, a trailing aircraft displaces itself laterally outside the primary wake vortex core, substantially reducing the probability of a damaging encounter.

Who May Use SLOP

SLOP is authorized for use by turbine-powered aircraft equipped with a Flight Management System (FMS) or other navigation system, such as GNSS, capable of executing an automatic offset function precisely. The equipment must be capable of maintaining the selected offset accurately. Aircraft not equipped with sufficient navigation accuracy should not attempt SLOP, because an imprecise offset provides no safety benefit and introduces additional uncertainty into the traffic picture.

The procedure applies in oceanic airspace and certain remote areas designated for its use. In practice, SLOP is routinely applied across the North Atlantic Track System (NAT), the Pacific, and other oceanic flight information regions (FIRs) where procedural separation governs. Some continental remote areas with limited radar coverage may also authorize SLOP — pilots should consult the applicable Aeronautical Information Publication (AIP) or the relevant NOTAM for the specific FIR to confirm applicability.

Operational Execution

Before entering oceanic airspace, the flight crew should brief SLOP as part of the oceanic entry checklist. The decision about which offset position to select — zero, one, or two NM right — should be made deliberately and randomly relative to what other crews might choose. Once selected, the offset is programmed directly into the FMS. Most modern FMS units have a dedicated OFFSET or LATERAL OFFSET page where the crew enters the offset distance and direction. The system then calculates and flies the offset track parallel to the assigned route.

During oceanic cruise, the flight crew should periodically verify that the FMS is maintaining the selected offset, particularly after any waypoint transitions, step climbs, or rerouting. A common practical caution — dependent on the specific FMS design and airframe rather than a codified FAA rule — is that some systems may clear the offset upon a direct-to entry or route modification, so crews should verify the offset remains active after any such change and re-enter it if necessary.

When approaching the oceanic exit point and transitioning back to a radar-controlled domestic environment, the offset must be removed before ATC resumes radar identification. ATC will expect the aircraft to be on its assigned route centerline when radar contact is established. Retaining an offset in domestic airspace where ATC is issuing instructions based on centerline positions creates a safety hazard and a compliance problem.

Why SLOP Matters

The statistical basis for SLOP comes from collision risk modeling developed by ICAO and incorporated into FAA oceanic operations guidance. Even with proper procedural separation, the probability of a collision in oceanic airspace is non-zero. Navigation errors, altimetry errors, and turbulence-induced altitude deviations all contribute to residual risk. SLOP directly attacks the lateral component of that risk by spreading aircraft across a wider lateral corridor, reducing the probability that a gross navigation error places two aircraft at the same point simultaneously.

Wake turbulence encounters in oceanic airspace are particularly insidious because they can occur without any visual warning, there is no radar to provide traffic advisories, and TCAS may provide only late warnings in certain closing scenarios. A structural upset from wake turbulence at FL390 over the mid-Pacific, hours from a diversion airport, represents an extreme risk. SLOP mitigates this at virtually zero cost — it requires only a few seconds of FMS programming.

From a regulatory standpoint, AC 91-70C (Oceanic and International Operations) provides detailed guidance on SLOP. Operators conducting Part 121 or Part 135 oceanic operations are expected to have SLOP addressed in their Operations Specifications (OpSpecs) and company procedures. ATP candidates should understand that SLOP is not optional guidance to be taken lightly — it represents an expected standard of practice for professional oceanic operations.

Key Numbers and Rules

Authorized offsets: Centerline (zero offset), 1 NM right, or 2 NM right only.
Left offsets: Never authorized under SLOP.
ATC notification: Not required — no clearance needed to initiate, maintain, or cancel a SLOP offset.
Position reports: Filed as if flying the assigned centerline; the offset is not mentioned in position reports.
Equipment requirement: Turbine aircraft with FMS or equivalent navigation system (such as GNSS) capable of flying a precise automatic offset.
Offset removal: Must be removed before entering radar-controlled airspace or as otherwise required by the applicable FIR procedures.
Randomization: Crews are expected to select their offset randomly — not to coordinate offsets with other aircraft.
Primary FAA source: AC 91-70C, Oceanic and International Operations.

Common Test Traps

Left offsets are never permitted. Exam questions sometimes imply that offsetting left is acceptable in certain circumstances — it is not. SLOP is right-only, always.
No ATC clearance is needed. Students sometimes confuse SLOP with a special clearance item. The offset is crew-initiated and requires no ATC coordination or notification whatsoever.
Offsets are not reflected in position reports. Position reports are made relative to the assigned track centerline, regardless of what offset is being flown.
FMS route changes can cancel the offset. A common practical trap on many FMS designs is that direct-to entries or route modifications can silently delete the programmed offset. Crews should verify the offset remains active after any route change, since this behavior varies by aircraft type.
SLOP must be removed before re-entering radar contact. Failing to remove the offset before domestic airspace entry is a compliance and safety issue — ATC expects the aircraft to be on centerline when radar identification is established.