ATP-3.3.4.6 air-to-air refuelling probe-drogue interface

What it is

What ATP-3.3.4.6 covers

ATP-3.3.4.6 is the NATO Allied Technical Publication that fixes the physical interface between a tanker aircraft's trailing drogue and hose and a receiver aircraft's refuelling probe, the probe-drogue method of air-to-air refuelling. Edition A, Version 1 was promulgated in June 2016 and is effective on receipt; the document does not describe itself as superseding an earlier edition. It is short and almost entirely dimensional: an introduction, a four-clause specification chapter with five further design notes, and four annexes of dimensioned drawings and a test-gauge tolerance table.

Its stated aim is "to provide the air-to-air refueling (AAR) systems requirements and to standardize the interfaces required to engage in probe-drogue refueling operations to facilitate AAR between aircraft of NATO forces." Participating nations "agree to use the probe-drogue system and interface requirements established in this NATO STANDARD and its ANNEXES."

Who it binds, and how

An Allied Technical Publication is not itself a contract or a law. It is content that acquires force through the STANAG that covers it: here, STANAG 3447, which the publication's own promulgation letter names as the record of "the agreement of nations to use this publication." A nation implements ATP-3.3.4.6 by instructing that future AAR equipment it develops, or has built under contract, will conform to the specifications here, and by identifying any exceptions for aircraft or equipment that already existed when the publication was promulgated. The document's operative language throughout is about what a nation instructs and identifies, not about a named Supplier or Acquirer role.

A company is drawn in only once a nation's instruction, or a contract or tender referencing this publication, requires the tanker or receiver equipment being built to conform to it. Within the document, the closest thing to direct address to a company sits in the annexes, which speak to whoever is doing the engineering: notes for "new receiver design" and "new tanker design," and guidance addressed to "receiver aircraft designers." Adoption is also visibly uneven rather than automatic: this edition's own record of reservations shows Germany unable to meet the universal pressure limit for one aircraft type, and France limiting its adoption to new equipment while excluding two clauses outright.

What it actually specifies

Chapter 2 works through the interface clause by clause, each tied to an annex or a figure. The mating dimensions of the reception coupling and the probe nozzle have to conform to the drawings in Annexes A and B (clause 2.1). A clearance envelope around the probe nozzle and probe mast installation follows Annex C, with receiver aircraft structure kept outside its hatched area and the tanker's drogue parts kept inside it (clause 2.2). The force to engage the nozzle in the coupling must not exceed 155 lbf at a head pressure of 10 psig (clause 2.3), and functional latch operation is verified with the Annex D test gauge, passed over the latches in both directions, with a maximum of 20 lbf (89 N) engaging and 50 lbf (223 N) disengaging (clause 2.4).

Fuel pressure is specified separately from the mechanical interface. Where a tanker can deliver more than 55 psig (379 kPa), it has to regulate steady-state delivery pressure downstream of the nozzle outlet to that limit at every flow rate down to zero flow (clause 2.5), and pressure surges from either aircraft must not exceed the other's design proof pressure (clauses 2.6 and 2.7). Two further note sections address legacy equipment rather than adding new requirements: clause 2.8 records that some existing tanker drogue systems need up to 30 cc/min flow to hold their regulated 55 psig, so a new receiver drawing less than that at top-off may see higher steady-state pressures, and that some existing systems allow up to 65 psig under no-flow or low-flow conditions; clause 2.9 notes that some legacy receivers (helicopters are given as an example) may need a lower interface pressure, that some legacy receiver probes are not compliant with this publication, and that some legacy receivers have a fuel system proof pressure of only 120 psig (828 kPa).

Annex C also carries design notes rather than binding limits: it records that its clearance figure does not account for relative movement between drogue and receiver during a mis-engagement, that a minimum of 12 inches (305 mm) is provided between the drogue's outer edge and the aircraft canopy once engaged, and that the figure is drawn for fixed-wing aircraft, with three of its dimensions not necessarily applying to rotary-wing design. None of this is offered here as guidance to act on; it is reported from the clause that states it, for a reader to take to the specification itself.

How conformance is checked

The document names no certification, audit or third-party assessment scheme, and no accredited or government body is described as checking anyone's conformity to it. What it describes instead is a nation checking its own equipment: clause 1.4 requires a nation to identify any exceptions to the specifications for aircraft or equipment that predate the publication, or that it later modifies, rather than an outside party making that determination. At the hardware level, the only verification step in the document is functional, the Annex D gauge test for latch operation. There is nothing resembling a certificate, and no sector scheme is named.

How it references other publications

ATP-3.3.4.6 is covered by STANAG 3447, which is what gives it force between nations. It names two related publications as background rather than as prerequisites: STANAG 3971, which covers ATP-3.3.4.2, the general air-to-air refuelling requirements document this one narrows to the probe-drogue interface, and STANAG 7191, which covers ATP-3.3.4.5, the sibling publication for the different boom-receptacle refuelling method. It also lists a set of US Military Specifications and Standards for individual parts (a hose assembly, a nozzle, a coupling and drogue-cone components), the DoD Joint Service Specification Guide JSSG-2001, and two Aerial Refueling Systems Advisory Group reports on pressure definitions and technical data; none of these sit in our catalogue, so they are named here rather than linked.

Getting the document

NATO publishes ATP-3.3.4.6 through the NATO Standardization Document Database at no charge. ComplyTrain does not sell it or host a copy; the NSDD listing for ATP-3.3.4.6 is the source.

Standards it references

• STANAG 3971Background
• ATP-3.3.4.2Background
• STANAG 7191Background
• ATP-3.3.4.5Background