ATP-3.3.4.7 air-to-air refuelling signal lights

What it is

ATP-3.3.4.7 is the NATO Allied Tactical Publication that fixes the signal lights a tanker aircraft shows to a receiver aircraft during hose and drogue air-to-air refuelling, for crews operating both with and without night vision imaging systems (NVIS). It sets the colours, number, position, intensity and flashing behaviour of the lights, ties each signal to a mandatory or advisory instruction for the receiver crew, and states when fuel flow stops or is stopping. Edition B, Version 1 was promulgated in October 2023; the printed reservations refer back to "the reservation in edition 1", so this is at least a second edition, though the text does not itself say what it superseded.

Like any Allied Tactical Publication, ATP-3.3.4.7 does not bind a company by existing. Participating nations agree, under STANAG 7215, to use the display, meanings and principles it sets out "as a reference," and a nation implements it domestically when it has "issued instructions that all future equipment developed or services contracted will be in accordance with the requirements detailed in this agreement." A national reservation printed in this edition's front matter argues the substance is really "more a matter of materiel engineering and technical requirements" aimed at "the acquisition and engineering community" that specifies and buys hose and drogue AAR systems, rather than tactical aircrew doctrine in its own right. In practice the document reaches a national air force through ratification, an engineering or acquisition team through a contract or specification that invokes it, and a tanker or receiver aircrew through the operating procedures it describes. France, Italy and the United States have each recorded specific reservations against parts of it, so whether it applies to a given fleet or programme is a question for that nation's own instructions, not a general one.

Colours, number and position of the lights

Without NVIS, two sets of lights are required, each containing a RED, AMBER and GREEN light of equal intensity, positioned symmetrically one set either side of the refuelling hose (clauses 2.1.1-2.1.4). The duplication exists for redundancy against a single light failure, to reduce disorientation, and to raise the odds a receiver crew sees a full set of lights when it has to move off the centreline (clause 2.1.3).

How the lights behave: on, flashing and dimmed

Each light is ON, FLASHING or OFF (clause 2.1.5), and flash rate differs by colour, with FLASHING RED faster than FLASHING GREEN. Dimming carries one deliberate exception: "for signals involving the use of FLASHING RED lights, the lights shall ignore the dimmed setting and use full brilliance," while RED ON and every AMBER or GREEN signal, flashing or steady, "shall respect the dimmed setting" (clause 2.1.6). After a FLASHING RED signal has shown for 10-15 seconds it reverts automatically to RED ON (clause 2.1.7), and after a receiver disconnects, a RED ON signal is provided automatically if the system is not yet ready for another contact (clause 2.1.8).

What each signal means, and which one wins

FLASHING RED cancels every other signal and is the breakaway signal, a mandatory instruction to carry out the breakaway procedure described in ATP-3.3.4.2 (ATP-56(D)). RED ON cancels AMBER and GREEN and covers non-emergency disconnect and "maintain astern" or "too close, move aft" instructions. AMBER cancels GREEN and covers "move forward into refuelling range" and a low fuel-flow condition; GREEN covers a higher fuel-flow state and lets the receiver crew remain in range. FLASHING RED, RED ON and FLASHING AMBER are all "mandatory instructions to the receiver crew"; only a FLASHING GREEN signal leaves the receiver crew free to "remain in the refuelling range appropriate to the circumstances" (clause 2.2.3). Fuel flow "shall be stopped or shall be stopping" whenever FLASHING RED, RED ON, FLASHING AMBER or AMBER ON is displayed (clause 2.2.4). Annex A also splits the AMBER and GREEN fuel-flow signals by a flow-rate threshold, though a footnote to that Annex cautions that "the aircraft with very low fuel on load rates of less than 50 gal per min may observe a Flashing Green Light through the whole refuelling period," so the flow-based split will not read the same way for every receiver type.

The NVIS version: a count of lights instead of a colour

Where NVIS makes coloured lights indistinguishable, Chapter 3 switches the coloured system off and replaces it with a count of lights - 3 for RED, 2 for AMBER, 1 for GREEN - carrying equivalent flashing, dimming, priority, breakaway and disconnect behaviour to Chapter 2. Positioning is more exacting here, because the receiver crew's task becomes counting rather than distinguishing colour: the spacing between the two light sets has to be greater than the spacing within a single set, specifically so a single light failure cannot be mistaken for a different, valid count (clause 3.3.4). Where the document describes what a receiver crew does about an odd, irregularly spaced count of lights, it is prescribing a flight-safety response for aircrew in that document, not advice this page is offering a reader.

What it does not cover

The standard "shall not apply to Boom Drogue Adapter systems" (clause 1.4.1), and it "shall not cover signal lights used for either formation control or rendezvous" (clause 1.4.2). Its terminology also comes from elsewhere: "terms written with the first letter being capitalised shall have the meaning given to them in STANAG 3971" (clause 1.4.3), the general dimensional and functional characteristics of the equipment come from STANAG 3447, and the light colours themselves are defined in STANAG 3224 and Mil-C-25250, not in this document.

Where nations have already disagreed

This edition's Record of Specific Reservations shows France excluding two specific aircraft fleets, Italy declining to retrofit its current fleet while endorsing the standard for future platforms, and the United States both disputing the document's classification as a tactical publication and declining to upgrade in-service AAR systems or apply it to newly purchased aircraft or services using older AAR equipment. The document itself notes the printed reservations "may not be complete" and points to the NATO Standardization Documents Database for the current list.

How it is assessed

ATP-3.3.4.7 names no certification body, notified body or audit scheme anywhere in the text. It is implemented, per clause 1.5, when a nation has "issued the necessary orders/instructions to the forces concerned" to put the signal lights, displays, meanings and principles into any new drogue system it procures or fields - a matter of national instruction and acquisition, not third-party assessment. Conformity in practice runs through whatever engineering and airworthiness acceptance process the procuring nation already applies to a new aircraft system, and through the specification a contract writes against this document. The document itself is free of charge from NATO, listed and served through the NATO Standardization Document Database.

Standards it references

• ATP-3.3.4.2Background
• STANAG 3447Background
• STANAG 3971Background