FMU-139 Does Not Perform as Expected

[ExNusquam]

Writing up a series of conditions that document the FMU-139 is not functioning correctly, and the avionics interfaces within the F/A-18C do not match expected behavior. Tracks are attached for all conditions.

 

Source for all of this is here: https://www.fighterpilotpodcast.com/post/understanding-the-fmu-139-and-its-employment-options - Source description "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."

This is a fantastic source that is unclassified, publicly released, and includes avionics depictions that is contemporaneous to our F/A-18C.

 

 

Condition 1: A/C Loaded with 1xLGB and 1xJDAM, each with an FMU-139 set to 10s ARM Delay and INST Function time

Expected Behavior: Both weapons should have either EFUZ w/INST/DLY1 as options, ARM function should give options for either 5.5s or 10s. Alternate: MFUZ Option with OFF/TAIL options. ARM function should give 6s/7s/10s/14s/20s options

 

Observed behavior: JDAM shows EFUZ w/INST/DLY1 as options, ARM function is not usable, JPF Page/Function was incorrectly available (Showing 14s Arm Time), however the weapon honored the 10s arm time (went High-Order after 12s TOF). Dud cues honored 10s time set on loadout page.

 

LGB incorrectly displayed EFUZ w/INST/DLY1/DLY2 options, no Arm Function.

 

Condition 2: A/C Loaded with 1xLGB and 1xJDAM, each with an FMU-139 set to 14s ARM Delay and 10ms Function time

Expected Behavior: Both weapons should have MFUZ (EFUZ not valid for arm times >10s) with OFF/TAIL, Arm function should give 6s/7s/10s/14s/20s options.  Weapon should execute 10ms delay on function. Weapon should go High-Order with specified function time with TAIL boxed. Alternate: if EFUZ must be used, weapon should go High-Order at specified function with DLY1, and on contact with INST selected.

 

Observed behavior: JDAM incorrectly shows EFUZ w/INST/DLY1, ARM function not usable, JPF Page/Function was incorrectly available (Showing 14s Arm time). Weapon went High-Order with INST selected (not correct for this Arm Time selection) but duded with DLY1 boxed.

 

Condition 3: JDAM with FMU-139 10s Arm Delay and INST Function Time, DSU-33

Expected Behavior: EFUZ Option with INST and VT1 as options. INST should function on contact and VT1 should function as airburst. Alternate: MFUZ with OFF/TAIL options. Airburst always functions.

Observed Behavior: EFUZ option incorrectly shows VT1/INST/DLY1. Arm Function is not usable. VT1 results in weapon dud (incorrect), INST executes airburst (incorrect), DLY1 duds weapon.

 

Condition 4: JDAM with FMU-139 14s/INST Plugged Extended TOF

Expected Behavior: With EFUZ option (FFCS Mode), JDAM should have TOF limit of either 60s or 240s. With MFUZ (FZU Mode) JDAM should have unlimited TOF.

 

Observed Behavior: JDAM fails to go high-order for TOF of 139s. Need further testing to verify what actual TOF limit is (but it's clearly not one of the expected ones)

Condition_1.trk Condition_2.trk Condition_3.trk Condition_4.trk

[ExNusquam]

I note that this got moved out of the Hornet forum (put there for specific avionics refs for the Hornet) - is there a status on this (reported, WAD, under investigation?)

[Muchocracker]

I've reviewed some of your test cases and the things you pointed out as unexpected behavior and here's some of my notes. 
 

On 7/23/2024 at 10:02 AM, ExNusquam said:

Condition 1: A/C Loaded with 1xLGB and 1xJDAM, each with an FMU-139 set to 10s ARM Delay and INST Function time

Expected Behavior: Both weapons should have either EFUZ w/INST/DLY1 as options, ARM function should give options for either 5.5s or 10s. Alternate: MFUZ Option with OFF/TAIL options. ARM function should give 6s/7s/10s/14s/20s options.

Couple of things to note with the ARM function. As you correctly pointed out it's basically non-functional in DCS as of right now. But there's some confusion that's gonna be caused if we don't narrow down which function the ARM option is serving depending on the initiator that's being loaded.

When the MK-112 initiator switch is loaded, the ARM button is in "FFCS" mode. Which gives 5 and 10 second arming delay times. More importantly the option selected here does change the arming delay on the bomb, as it's connected to the jet with a COAX cable. This makes the FMU-139 show up with EFUZ in the SMS

The 6/7/10/14/20 second arming delay is for the "FZU" mode of the ARM button when the bomb is loaded with the FZU-48 initiator. Contrasted with the 122 the ARM option does not change anything on the bomb and is only for matching the faceplate with the SMS so the DUD cues are proper. The FZU is a mechanical switch that is energized by a turbine after release. This makes the FMU-139 show up with MFUZ/TAIL in the SMS


It's hard to know which one ED is basing all of their avionics functions on as there's some things that are using the mk-122 functionality in the avionics. But with the bomb fusing settings themselves they would be set up for the FZU-48. Case in point; If the bombs were set up for the Mk-122 we would have no arming delay setting as the SMS would automatically override any setting used in on the low drag faceplate. And any highdrag deliveries would cause the arming delay to be overriden to 2.6 seconds. 

Idk man, for ED to fix this issue it's gonna be complicated as they have to maintain compatibility across all modules that potentially use different fuses as well as initiators (mk-122 is navy specific). Whether than means they make the system module specific so they can do this or they add initiators to the fusing menu depends on them. 

 

On 7/23/2024 at 10:02 AM, ExNusquam said:

 JPF Page/Function was incorrectly available (Showing 14s Arm Time), however the weapon honored the 10s arm time (went High-Order after 12s TOF)

i've not found any evidence to suggest that the JPF page goes away if the JDAM is not loaded with the FMU-152. So unless you have some then i wouldn't consider this relevant nor any of the information it provides as it's not connected to anything. 

 

On 7/23/2024 at 10:02 AM, ExNusquam said:

Condition 3: JDAM with FMU-139 10s Arm Delay and INST Function Time, DSU-33

Expected Behavior: EFUZ Option with INST and VT1 as options. INST should function on contact and VT1 should function as airburst. Alternate: MFUZ with OFF/TAIL options. Airburst always functions.

Observed Behavior: EFUZ option incorrectly shows VT1/INST/DLY1. Arm Function is not usable. VT1 results in weapon dud (incorrect), INST executes airburst (incorrect), DLY1 duds weapon.

This one i have reported before as well interally, but i forgot that this FPP document directly calls it out the DLY1 is replaced with VT1. Will add to that report. 

 

 

On 7/23/2024 at 10:02 AM, ExNusquam said:

Condition 4: JDAM with FMU-139 14s/INST Plugged Extended TOF

Expected Behavior: With EFUZ option (FFCS Mode), JDAM should have TOF limit of either 60s or 240s. With MFUZ (FZU Mode) JDAM should have unlimited TOF.

 

Observed Behavior: JDAM fails to go high-order for TOF of 139s. Need further testing to verify what actual TOF limit is (but it's clearly not one of the expected ones)

 

Honestly, it should just default to the newest available one that there are detailed documents of. 

Edited August 9, 2024 by Muchocracker

[ExNusquam]

Quote

Idk man, for ED to fix this issue it's gonna be complicated as they have to maintain compatibility across all modules that potentially use different fuses as well as initiators (mk-122 is navy specific). Whether than means they make the system module specific so they can do this or they add initiators to the fusing menu depends on them. 

I absolutely agree, and would happily accept some compromises for the sake of compatibility with other modules. However, we have excellent sources for how it should work for the Hornet - hence why I'm reporting it. I'd like bombs that at least something predictable (like go high-order with reasonable settings applied).

[El Chapo]

One more question regarding FMU-139 and their use with Snake Eyes (and it's common to all aircraft, not just the Hornet): how am I supposed to setup the arm delay for my bombs for both low drag and high drag cases?
Currently, if you select the Snake Eye with FMU-139, you only have high drag fuze options. But if I decide to launch it as a low drag bomb (e.g. FF mode in the Hornet) then I didn't setup my arming delay...
Normally, on the FMU-139, we should always be able to set-up both the LOW DRAG and HIGH DRAG options, and the bomb knows which one to use depending on the initial deceleration.

If I were ED, I would turn the fuze setting page into a visual representation of the fuze, and let people "turn the screws" as it would be done for real on the jet. Same for laser settings, and it would avoid being able to set LASER codes that can't be physically selected on the bombs.

Thanks

[Muchocracker]

16 hours ago, El Chapo said:

One more question regarding FMU-139 and their use with Snake Eyes (and it's common to all aircraft, not just the Hornet): how am I supposed to setup the arm delay for my bombs for both low drag and high drag cases?
Currently, if you select the Snake Eye with FMU-139, you only have high drag fuze options. But if I decide to launch it as a low drag bomb (e.g. FF mode in the Hornet) then I didn't setup my arming delay...
Normally, on the FMU-139, we should always be able to set-up both the LOW DRAG and HIGH DRAG options, and the bomb knows which one to use depending on the initial deceleration.

If I were ED, I would turn the fuze setting page into a visual representation of the fuze, and let people "turn the screws" as it would be done for real on the jet. Same for laser settings, and it would avoid being able to set LASER codes that can't be physically selected on the bombs.

Thanks

Only thing you can really do is put on the 904E4 on the nose and set it up for inst impact fusing. Pretty much answerd it yourself, we can't set the FMU-139 properly at all because they made it EITHER low drag or HIGH drag faceplate selectable based on the bomb you put it in.

On 8/11/2024 at 8:11 PM, ExNusquam said:

I absolutely agree, and would happily accept some compromises for the sake of compatibility with other modules. However, we have excellent sources for how it should work for the Hornet - hence why I'm reporting it. I'd like bombs that at least something predictable (like go high-order with reasonable settings applied).

All we can do is make our case and make noise about it then hope ED puts significant resources into correcting.