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Understanding the FMU-139 and Its Employment Options

This unclassified article was originally penned in 2008 for inclusion into a classified Navy tactics periodical. Various grammatical changes have been made to change the tense of the text from what was expected to happen in 2008 to what is now known over a decade later.


As any child who has ever played with matches can attest, the need to control combustible materials is vital to safely perform a pyrotechnic event. While the penalty for a child’s carelessness may be a couple of burned fingers and a parent’s scorn, recklessness can be catastrophic in more volatile applications, such as when explosives are involved.

Of course, this revelation is not new. In 10th century China, the need to delay ignition in fireworks led to history’s first recorded concept of a “fuse” as gunpowder was loosely loaded into lightweight paper providing a simple, inexpensive solution.

In the eighteenth century, artillery projectiles were equipped with metal or wooden plugs containing paper whose burn rate was shown by its color. A black fuse burned an inch every two seconds, red burned three inches, green four, etc. Gunners would judge the firing distance, calculate the projectile’s time of flight, then select the appropriate fuse, cutting one shorter if necessary before lighting it and firing the shot.

Today, firework fuses are little changed from their Chinese ancestors; however, in the realm of modern aerial combat, the technology incorporated into air-to-ground (A/G) ordnance is radically advanced from simple paper and gunpowder. In fact, as a matter of semantics the spelling differs to capture the distinction between the two. According to Wikipedia, the term fuse describes a simple pyrotechnic detonating device like the cord on a firecracker, whereas fuze is used to describe a more complicated ignition device incorporating mechanical and/or electronic components. The latter is the subject of this article.

U.S. Navy and Marine Corps inventories contain hundreds of different types of fuzes. Some weapons, such as the AGM-154 Joint Standoff Weapon (JSOW) and AGM-65E Maverick, incorporate fuzes into the internal electronics and require little aircrew interaction.

Ordnance built around Mk 80 / BLU-series warheads, however, may be outfitted with more than one type of fuze—each with distinct features. One example is the venerable M904 mechanical nose fuze. Easy to install and simple to use, the M904 was in service for 30 years; however, its significance faded when aerial tactics shifted from reliance on numerous general purpose (GP) “dumb” bombs to fewer precision munitions such as GPS and laser-guided munitions, which are not compatible with the M904. This limitation combined with the M904’s age, which revealed impellor gearing issues posing safety and reliability concerns, caused Navy leadership to discontinue the program. The M904 was scheduled to be phased out of the inventory by FY2009.

A close up of the M904 fuze.

The FMU-143 series of electronic tail fuzes are designed specifically for hard target penetration and are used only with the BLU-109 and BLU-116 2,000 lb-class warheads affiliated with the GBU-31A(V)4/B Joint Direct Attack Munition (JDAM) and GBU-24 Paveway III series laser-guided weapons (LGWs). The FMU-143 is externally powered by a FZU-32, features only one factory-set functioning delay (60ms), and offers two arm times of which only the higher is authorized for the U.S. Navy and Marine Corps.

The FMU-139 electronic fuze series has been in service with the U.S. Navy, Marine Corps, and Air Force for over 25 years. This versatile device is employed in a wide array of A/G ordnance such as high- and low-drag GP bombs, Paveway II LGWs, and all JDAM variants except the GBU-31A(V)4/B. The FMU-139 may be employed as the sole detonation source at weapon impact or paired with a proximity sensor to facilitate a low altitude air burst.

With the demise of the M904, the limited applicability of the FMU-143, and the FMU-152A/B Joint Programmable Fuze languishing in testing, the FMU-139 will undoubtedly remain the most prolific and indispensable A/G fuze in the TACAIR inventory for many years. The intent of this article is not to delve into the engineering marvel that is the FMU-139 in agonizing detail, but rather to provide a useful overview of its capabilities, limitations, and employment considerations with a distinct focus on user interface. The term “authorized”, whenever used in this article, applies specifically to U.S. Navy and Marine Corps F/A-18 operations.


Several variants of the FMU-139 exist with each successive model improving on the previous. The FMU-139/B was a developmental prototype that never became operational despite being depicted in the fuze inspection section of both the F/A-18A-D and E/F Tactical Manual Pocket Guides.

The FMU-139A/B reached initial operating capability (IOC) in 1986 and functioned similarly to succeeding models but contained an older explosive booster known as CH-6. In 1996 a US Marine Corps AV-8B Harrier was destroyed in an FMU-139A/B early burst mishap later attributed to decaying CH-6. As a result, a Mk 3 arming wire was incorporated as an added mechanical safety feature. The Mk 3 was routed from the pylon’s BRU-32 to the fuze’s arming wire housing, physically preventing the fuze from arming until weapon release. Another outcome of the mishap investigation was the CH-6 was replaced with PBXN-7, resulting in the FMU-139B/B. The FMU-139A/B was phased out of service in 2002.

Besides the newer explosive booster, the FMU-139B/B encompasses manufacturing and technological improvements which eliminate the Mk 3 arming wire. The B/B and all preceding variants contain an electrical capacitor rated for 60 seconds when charged by the employing aircraft. The FMU-139B/B was expected to remain in service until either the ten-year in-container shelf life expired or the inventory was depleted.

The FMU-139C/B benefits from numerous manufacturing improvements over previous variants but is otherwise functionally identical to the B/B with the significant exception of an enhanced capacitor rated for a minimum of 240 seconds. The C/B was expected to IOC in early 2009 and is the assumed variant anywhere the term “FMU-139” is used in the remainder of this article.

The FMU-139(D-2) is a dummy fuze containing no explosives and is intended for ground training and other inert applications only.

Future FMU-139 variants currently in development are expected to feature hardened construction for penetration applications, which should eventually replace the FMU-143. A serial interface is planned similar to the FMU-152 which, with the appropriate aircraft software, will allow various arm time and other selections via the STORES page.

How It Works

The FMU-139 is considered an “electro-mechanical” fuze, meaning both electrical power and mechanical action are used for the fuze to operate. The flow of electrons performs fuze arm timing, detonation, and automatic selection of high drag arming time when a retarded delivery is sensed. Mechanical action unlocks and aligns the detonator and booster charges at the selected arm time which allows the fuze to later detonate. The FMU-139 does not contain its own internal source of electrical power but relies on an externally produced supply of direct current (DC) provided by either the aircraft at release or a ram-air turbine generator mounted in the warhead’s charging well. The fuze will function differently depending on the electrical power provided, a frequently misunderstood subtlety that will be explored in greater detail below. A small amount of PBXN-7 explosive booster is incorporated to cause high order detonation of the warhead. This feature makes the FMU-139 a category “A” explosive device necessitating handling by only qualified ordnance or explosive ordnance disposal (EOD) personnel.

Except for the nomenclature, the FMU-139B/B and C/B faceplates are identical (Figure 1). Each contains two rotary switches which allow selection of three fuze features. The “Low Drag Arm Time” switch has multiple options available for use in free fall deliveries but only the X, 10, 14, and 20 positions are authorized. As the slash in its title implies, the “High Drag Arm / Delay” rotary switch serves two purposes: selection of a high drag arm time and a fuze functioning delay. Of the four available high drag arm times only 2.6 seconds is authorized, rendering this feature irrelevant. Instead, the switch allows selection of one of four functioning delays: INST (instantaneous, i.e. no delay), or 10, 25, or 60 milliseconds. The white “interlock” button is inconsequential as it is used when selecting the 2.0-second high drag arm time, which is not authorized. Finally, the arming wire housing in the center of the faceplate contains a black and red-striped “gag rod” which extends during the arming process. A safing pin and “REMOVE BEFORE FLIGHT” tag may be installed in the housing to physically prevent the fuze from inadvertently arming during handling but are normally removed before aircrew become involved.

How We Use It

When tasked to destroy a ground target from the air, one of the first steps in the mission planning process is to analyze the target in an attempt to determine how to best achieve the desired damage. The “weaponeering” process results in a weapon-to-target match that specifies not only the best weapon but also terminal parameters such as impact angle, velocity, and whether penetration or an air burst is needed. Aircrew use this and other mission planning factors to determine specific target area tactics and to request the applicable weapon configuration from the ordnance department.

If an M904-equipped GP bomb is called for, the weapon and fuze arrive at the aircraft separately where ordnance personnel first load the weapon, then mount the M904 and attach the appropriate wiring.

An FMU-139, conversely, is installed as part of the weapon assembly process in the ship’s magazine (afloat) or at station weapons (ashore). Ordnance personnel position it in the warhead’s tail fuze well (mounting in the nose fuze well is not authorized), connect it to the warhead’s rear conduit, and secure the fuze in place with a threaded closure ring. The desired low drag arm time and functioning delay are selected during assembly but may be altered prior to takeoff if the mission changes. Ordnance personnel assemble the necessary components for the type of weapon ordered, then to provide the FMU-139 electrical power, install into the warhead’s charging well either a Mk 122 arming safety switch or FZU-48 bomb fuze initiator. Aircrew determine which component should be used based on the planned target area tactics.Mk 122. The Mk 122 arming safety switch serves two purposes: it acts as a radiation hazard (RADHAZ) shield preventing unwanted electromagnetic radiation from reaching the FMU-139, and it facilitates electrical connectivity between the aircraft and the FMU-139. The Mk 122 is equipped with an arming lanyard and a coaxial cable. The arming lanyard, shorter and usually red, attaches to the BRU-32’s center positive arming latch and performs the RADHAZ duties. As long as this lanyard is in place, all electrical signals—wanted or otherwise—are barred from passing through to the FMU-139.

The black coaxial cable connects to the electrical fuze connector receptacle in the BRU-32 and facilitates the connectivity between the aircraft and the fuze. The wires are different length so that the Mk 122 may provide continuous RADHAZ protection from weapon installation until the first few inches of fall after release. Once the arming lanyard disconnects, the Mk 122 switch closes allowing a DC voltage from the aircraft to be applied through the coaxial cable and the rear conduit to the fuze. The FMU-139 determines the aircrew-chosen programming options based on the voltage’s magnitude and polarity. The voltage charges the capacitor which provides the necessary electrical power throughout the time of fall (TOF). The coaxial cable then disconnects as the weapon continues to fall with a fully programmed, self-sufficient fuze.

The FMU-139 / Mk 122 combination requires a tail fuze code selection of ‘3’ on the F/A-18 stores management panel (SMP) and is referred to as the “FFCS Mode” after the fuze function control set (AWW-4), which provides the DC electrical signal on Lot 18 and below Hornets equipped with the AN/AYQ-9 stores management set (SMS). The signal is generated by the AN/AYK-22 on Lot 19 and above Hornets and Lot 24 and below Super Hornets, and by the AN/AYK-PPC (which stands for PowerPC) on Lot 25 and above Super Hornets.

FFCS Mode. One of the principal advantages of selecting a Mk 122 for use with the FMU-139 is the cockpit programming options it affords. As depicted in Figure 2, the STORES page will display ARM and EFUZ options at pushbuttons (PBs) 1 and 3 respectively. Depressing PB1 yields low drag arm time options of either 5.5 or 10 seconds at PBs 5 and 4. Note the FMU-139 faceplate (Figure 1) depicts a low drag option of 10 seconds but not 5.5. This is coincidental as the fuze is programmed to ignore the low drag arm time rotary switch position and arm at the aircrew selected time of either 5.5 or 10 seconds after release. Thus, to avoid any possible confusion, ordnance personnel are required to set this switch to the ‘X’ position. In the case of a GP bomb with a retarding fin assembly, if a high drag delivery is chosen the arm time will default to 2.6 seconds with a leading asterisk (*), showing the SMS has overridden the aircrew’s input.

The EFUZ option at PB 3 allows functional control of the FMU-139. OFF (PB 5) may be thought of as “safe” because when selected, no voltage is applied to the Mk 122, thus the capacitor is never charged and the weapon duds. INST (PB 4) commands the FMU-139 to detonate instantly either at impact or when directed by a proximity device, regardless of the high drag arm time / delay switch setting. DLY1 (PB 3) commands detonation at the pre-flight selected functioning delay following impact or proximity signal receipt. Since in the FFCS Mode an instantaneous detonation is always cockpit selectable, aircrew should not request the high drag arm time / delay rotary switch be set to 2.6 / INST on the FMU-139 faceplate unless using a proximity sensor. Instead, one of the three other settings should be selected; either as determined during the weaponeering process to have the best effect against the planned target or to provide flexibility should dynamic weaponeering on emerging targets be required in flight.

One drawback of using a Mk 122 is the longest available arm time is 10 seconds. Depending on delivery parameters, this might exclude a straight and level safe escape maneuver option, which could be problematic in certain circumstances (a division dropping JDAM at night, for example). Another limitation is the TOF must be planned for no more than 60 seconds when using the FMU-139B/B. With the FMU-139C/B, however, deliveries can now be planned for up to a four-minute TOF.

FZU-48. The FZU-48 bomb fuze initiator is a small air turbine generator contained within a cylindrically-shaped metal housing that mounts in the warhead’s charging well. A FZU-61/B lanyard attaches the FZU-48 to the BRU-32’s aft zero retention force (ZRF) solenoid to allow optional cover assembly opening. Once open, a minimum airflow of 140 knots is required to generate enough electrical power to arm the fuze; however, 250 KCAS is the published minimum for F/A-18 operations. The FZU-48 provides continuous power throughout an unlimited TOF. A tail fuze code selection of ‘6’ is required for LGWs and ‘7’ for JDAM. The FZU-48 / FMU-139 combination is referred to as the “FZU Mode”.

FZU Mode. Unlike the FFCS Mode, no direct electrical connection exists between the aircraft and the FMU-139 in FZU-48 equipped weapons. Aircrew in-flight options are therefore limited to either arm the weapon or not.

Figure 3 depicts the cockpit symbology for a FZU Mode JDAM. The MFUZ option at PB 4 may cause confusion as it is also used for weapons equipped with mechanical fuzes, such as the M904. This is merely an unfortunate coincidence. Since the fuze in this case is the FMU-139, MFUZ might be more accurately thought of as “manual fuze” control. Selecting it presents the options OFF and TAIL at PBs 5 and 4. OFF is the equivalent of “safe”; when selected, the ZRF solenoid releases the FZU-48 lanyard at weapon release causing the cover assembly to remain closed. Since electrical power is never generated, the weapon duds. Selecting TAIL, on the contrary, is the equivalent of “armed” as the ZRF now retains the lanyard, causing the FZU-48 to open and generate electrical power for the FMU-139 to operate.


The DC voltage provided by a FZU-48 is much lower than that supplied from an aircraft through the Mk 122. The FMU-139 interprets this as a cue to use the preflight-selected low and high drag arm times and function delay settings on the faceplate. The authorized low drag arm times are 10, 14, and 20 seconds. The 4, 6, and 7 positions are not authorized and the ‘X’ will result in a dud. The only high drag option remains 2.6 seconds and the functioning delay must be set as desired.

For JDAM (Figure 3), the ARM option at PB 1 reveals the options 6, 7, 10, 14, and 20 at PBs 5 through 1. Selecting the arm time corresponding to the actual selection on the fuze faceplate allows accurate dud cue symbology in the cockpit. Selecting any other setting does not change the actual arm time but merely presents inaccurate dud cueing. Curiously, FZU Mode LGWs do not incorporate dud cueing unless OFF is the selected MFUZ option. For normal releases when TAIL is selected, it is up to the aircrew to determine whether the TOF will be sufficient for fuze arming.

It is important to note that confusing the FZU and FFCS Modes may lead to less than optimal results. In the FFCS Mode the low drag arm time must be set to ‘X’ but in the FZU Mode it must not be set to ‘X’ or the weapon will dud. Regrettably, the current but badly outdated F/A-18A-D and E/F Tactical Manual Pocket Guides do not helping matters as the FMU-139 inspection section of both states, “LOW DRAG ARM TIME SWITCH SHOULD ALWAYS BE IN ‘X’ POSITION”, in blatant disregard of when a FZU-48 is used.

Finally, as previously mentioned, it is senseless to select an INST functioning delay in the FFCS Mode unless using a proximity sensor because an instantaneous detonation is always cockpit-selectable. In the FZU Mode, however, INST should be selected if the weaponeering dictates.

Employment Considerations

The FMU-139 is compatible with a wide variety of A/G weapons delivered in various conditions such as level lay down, high or low angle dive bombing, high or low drag, and in singles or as a stick of multiple weapons. The maximum authorized release airspeed of an FMU-139 equipped weapon from an F/A-18 is 600 knots. The fuze does not impose a minimum speed restriction in low drag applications; however, a minimum of 400 knots is required in retarded deliveries for the electronics to sense the requisite 4 G deceleration, thereby invoking the 2.6-second high drag arm time. If less than 4 Gs of deceleration is sensed in a high drag-selected, FFCS Mode weapon, a 10-second default arm time will be invoked—most likely resulting in a dud in low altitude deliveries.

When planning dive deliveries of more than one Mk 122-equipped GP weapon, aircrew may find the 5.5-second arm time limits the maximum stick length that may be planned as a precaution against early burst. The concern is a long stick length results in the last weapon released being too close to the aircraft when the first weapon released arms. If that first weapon inadvertently detonates immediately upon arming, it could start an early burst chain reaction of the following weapons (even if they have not yet armed). If the last weapon released detonates close to the aircraft, the result could be catastrophic. The best solution is typically to elevate the delivery altitude so a 10-second arm time may be used. Alternatively, a shorter stick length may be planned, but if the stick is shorter than the weaponeering-derived length, aircrew run the risk of not achieving the desired damage on the target.

Conversely, trying to plan a release of multiple closely spaced weapons can also be problematic as now early bursts due to bomb-to-bomb collision must be considered. Each of the various weapon components being used (the particular fin assembly, a proximity sensor, etc.) impose various minimum release interval restrictions which may force a minimum stick length. This can again be an issue depending on weaponeering requirements.

Fortunately, WASP considers these and many other issues, allowing planners to manipulate the attack parameters until a suitable delivery can be found.

Proximity Sensors

When an A/G weapon detonates upon ground impact, the earth tends to absorb much of the warhead’s blast wave and fragmentation. A proximity sensor causes warhead detonation just above the ground, maximizing the blast and fragmentation affects on the target. Two proximity sensors are authorized for F/A-18 operations. The Mk 43 is an older, technologically inferior device. It is not compatible with JDAM and is expected to be retired soon; therefore, the Mk 43 will not be discussed in this article.

The DSU-33, on the other hand, is a technological marvel, incorporating Doppler radar processing and electronic protection. It is the most commonly employed proximity sensor in use, particularly with JDAM.

DSU-33. The DSU-33A/B reached IOC with the U.S. Air Force in 1994 but is not authorized due to RADHAZ concerns in the carrier environment. The DSU-33B/B achieved IOC in 1999 and remains in service with the follow-on DSU-33C/B (Figure 4). The latest variant, the DSU-33D/B, incorporates upgrades which reduce costs and improve height of burst accuracy, among other improvements. The U.S. Air Force has taken delivery but there is currently no plan for the naval services to do so.

The DSU-33 is a battery-powered, all-weather proximity sensor using Doppler radio frequency ranging to determine its height above the surface, similar to an F/A-18’s radar altimeter. When the DSU-33 determines its height to be 20 feet above a surface, it sends an electrical command for the fuze to detonate. It is suitable for both high and low drag deliveries over any surface with a delivery airspeed range of 250 to 700 KCAS. The DSU-33 mounts into a warhead’s nose fuze well and requires an armament code of ‘B’. It is not considered a fuze since it contains no explosive material. It does, however, communicate with electrical tail fuzes, specifically the FMU-139, via the forward and rear conduit.

When discussing this proximity sensor some F/A-18 software manuals improperly reference the DSU-30, which was a prototype of the DSU-33 that never became operational. Except for the nomenclature, aircrew should regard the information as accurate for the DSU-33.

Adding a DSU-33 to a weapon configured in the FFCS Mode causes the SMS to replace the EFUZ option DLY1 at PB 3 with VT1. OFF and INST functionality does not change but now aircrew trade the penetration option for an airburst. In the FZU Mode aircrew have no say in whether or not the DSU-33 functions. If the FZU-48 opens and power is present inside the weapon, the DSU-33 will always operate. In either case, when the FMU-139 receives a DSU-33’s signal to detonate it does so after waiting the high drag arm / functioning delay-selected time. That is, if the functioning delay is set to INST, the fuze detonates immediately upon the proximity sensor’s signal. If the selected functioning delay is 25 msec, the fuze waits 0.025 seconds after the DSU-33’s signal before detonating. This feature provides a jungle penetration capability.

Jungle Penetration. In the Vietnam conflict, military planners and strike aviators quickly learned a thick jungle canopy negated the affects of a proximity sensor as the tree tops triggered the device and partially shielded the targets below. To counter this, a delay was implemented so the proximity sensor would detect the trees but detonation would be postponed until the weapon traveled below the tree tops. As one might expect, it was an imprecise science as the weapon’s terminal parameters and the tree height and thickness varied on nearly every mission.

With the conflicts of the past 30 years being mainly in desert climes, the art of achieving an airburst below a thick jungle canopy has been largely lost. However, with regions such as the Philippines, Venezuela, and the horn of Africa ranking high on the United States’ areas of interest, this skill set may need to make a timely comeback. Fortunately, all it takes is a little high school trigonometry combined with knowledge of target area tree heights and thicknesses, and a precise JDAM air burst can be planned.

The first step is to determine how long it will take a weapon to travel from 20 feet above the tree canopy (when the DSU-33 commands detonation) to just below the tree branches.

Figure 5 shows a right triangle where C is the tree canopy thickness, A is the weapon’s impact angle, and D is the distance traveled. Trigonometry tells us the sine of an angle in a right triangle equals the opposite side divided by the hypotenuse. That is:

Assuming a terminal velocity (V) in feet per second (fps), the time it takes a weapon to detect the jungle canopy and travel below it is:

As a practical example: imagine a target area contains trees 35 feet tall with a canopy 5 feet thick. Suppose JDAM weaponeering produced terminal parameters of an 85 degree impact angle and 1,100 fps velocity. Plugging these numbers into the equation above yields the following:

Hence, the JDAM will take nearly 23 msec seconds to detect the jungle canopy and travel below it. Selecting the 25 msec functioning delay on the FMU-139 in either the FZU or the FFCS Mode will provide the necessary delay; the 10 msec setting does not. To decide if the 60 msec setting is a good option we must perform one more calculation to determine how long it will take the weapon to hit the ground, which is not the goal. The trigonometry in this endeavor is nearly identical to the steps taken above, except now we must consider the tree height (T) and the distance to the ground (G), as depicted in Figure 6. As before:

Therefore, the time it takes the JDAM to strike the ground is:

Referencing our example above, the weapon takes 0.050 seconds (50 msec) to reach the ground from when it first detects the tree tops. Thus, the 60 msec functioning delay is not a good choice as the weapon would impact the ground before the delay expired.

Table 1 lists a summary of the applicable functioning delays which should be selected for the impact angles and velocities shown assuming 35-foot tall trees with canopies 5-feet thick. By simply performing a little “plugging and chugging,” a squadron A/G Weapons Tactics Officer (WTO) can easily construct a similar table in advance of the command entering a theater of operations where lucrative targets may seek protection under a jungle canopy.

Table 1: FMU-139 Jungle Penetration Functioning Delay Settings Assuming 35-Foot Tall Tree with 5-Foot Thick Canopy

Of course, it goes without saying that, with a DSU-33, a functioning delay should only be selected on the FMU-139 when jungle penetration is desired in appropriately wooded target areas. Over water, deserts, or other target areas devoid of trees, selecting a delay may result in a ground detonation. INST should be selected to facilitate an airburst in those areas of operations.


The FMU-139 served the U.S. Navy, Marine Corps, and Air Force admirably for over two decades and is sure to remain the most commonly used A/G fuze for many years to come. Aircrew must have a firm understanding of the various ways to employ an FMU-139, with the associated benefits and limitations, to ensure adequate effectiveness in what remains the priority mission in every current theater of operations: air-to-ground attack. Table 2 offers a summary of FMU-139 operations in both the FFCS and FZU Modes, with and without the use of a DSU-33 proximity sensor.

Table 2: FMU-139 Summary (Times in Seconds, Airspeeds in KCAS)

(1) Airburst will always result

(2) LGWs use '6', JDAM use '7'

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