Tailhook Topics

by Tommy H. Thomason

Thursday, May 30, 2024

Modex Number

 This is a work in progress but I'm pretty sure of the basics...

The number on the side of the nose of U.S. Navy carrier-based airplanes is often referred to as the modex number (I don’t know of a USAF equivalent). In fact, strictly speaking, it is only one element of the modex number and is in fact, more accurately the “Unit Aircraft Numeral” [MIL-I-18464 (Aer)] or more colloquially, side number.

The "NH" on the vertical fin of this F3H-2N in 1960 is the Unit Identifier or tail code (for posts discussing tail codes, click HERE and HERE)  and the number under it is the last four digits of the Bureau Number, also known as a call number. Note that some units that were only assigned a few of a particular type of aircraft typically used the last three digits (occasionally four in the event of a duplication of the first three) of the BuNo for a side number on the nose.

The term modex apparently didn't exist until the early 1960s, perhaps coincidental with the introduction of the Grumman E-2A with an advanced Airborne Early Warning radar system that used the octal number system for computations like civil ATC's Mode 3 use of a four-digit octal code (4096 possible combinations, no 8s or 9s). It therefore resulted in the elimination of side numbers with the digits 8 and 9 (some arguably knowledgeable experts believe that this was also the result of the introduction of an early maintenance reporting system with a similar limitation). At some point thereafter it became fashionable but incorrect to refer to an aircraft’s side number as the modex number even though it was only part of it and even refer to a side number an aircraft in a picture taken before the early 1960s as a modex.

The Modex number was actually a four-digit code used to identify a specific aircraft by means of its Mode 2 Identification Friend or Foe (IFF) capability. According to Jan Jacobs, a former F-4 and F-14 RIO, it was not resettable by the pilot like the Mode 3 code, which was assigned by civil Air Traffic Control for radar aircraft identification. The first number was peculiar to the Air Wing that the squadron was assigned to and the second, third, and fourth digits were the aircraft’s side number. For example, Jan wrote “Air Wing 2's unique code was "7" so the CAG bird from VF-21 would have a 7200 Mode 2 squawk. The skipper's bird would be 7201. The AWG-10 radar in the F-4J had the capability of reading Mode 2 or 3 squawks by using the APX-76 IFF interrogator system. It was very handy in finding your tanker overhead the ship when there were 2 or more tankers on station. The RIO would set in the tanker's (modex) and the aircraft's distance and azimuth would show up as a double-bar on the radar scope.”

Jan goes on to describe why the side numbers never included the digits 8 or 9:  “The 50's/60's-era IFF systems used 8-bit data systems (Base 8). Digits available were 0 through 7 (a total of 8 bits). In this system, 8 and 9 did not exist as 1-digit numbers. Mode 2 used this system so no aircraft that were in a tactical air wing could use 8 or 9 in their (side numbers) because Mode 2 did not support it. You'll see RAG aircraft and other Navy aircraft with 8 and 9 in their side numbers, but they were not in a tactical air wing.”

Presumably the limitation of the first digit to 8 numbers (if 0 is included) was not a problem since there were never more than a few air groups flying in one geographical area.

There is one somewhat cryptic modex statement in the subsequent  Paint Schemes and Exterior Markings document, MIL-STD-2161A dated 1 May 1993: “MODEX numbers applied to aircraft with the tactical paint scheme shall be FED-STD-595, color number 36081…”. There is no definition of “MODEX number” in that Standard Practice document but there might be in subsequent revisions. More relevant and not inconsistent with this statement, assuming that it only applies to the tail code as well as the side number, is this Call Sign Requirements statement in NATOPS General Flight and Operating Instructions dated 23 November 2009 (it may not be current): Call sign selection for cross-country flights shall be made in accordance with DoD FLIPS. It is strongly recommended that squadron modex (NJ213, DB214) be used in flight planning. If the use of tactical/squadron call signs is necessary, call signs shall be the approved JANAP 119 call sign for the unit concerned. Abbreviations or contractions of these call signs in not authorized.

n.b. I had thought that the typical call-sign used in civil-aviation circumstances was the word Navy or Marine as appropriate followed by the last three or four digits of the Bureau Number.

The practice by some to caption a picture taken after the mid 1960s referring to the combination of the tail code letter(s) and side number (in that order) as a modex would be legitimized by this NATOPS statement but an anachronism for one taken earlier.

 I have yet to find an explanation of the origin of the term Modex, other than it may hark back to the post-war "Mark X" IFF system that could identify a specific aircraft as opposed to simply friend or foe. TACAN systems have a Mode X and Y but those denote two different frequency ranges.

Knowledgeable (e.g. not Wikipedia) comments, additions, and corrections welcome.

Wednesday, April 24, 2024

F-111B Self Boarding

There were few exceptions to a Navy requirement that carrier-based aircraft be self boarding, i.e. a separate ladder was not required. For more background on this, see:



The F-111B was not an exception:

Note that the bottom step is wide enough for both feet. Not obvious in the above illustration is the second step, a peg extended from the forward side of the brace going from the fuselage to the aft end of the bottom step.

 Note the inflight refueling probe location in the illustration above is the original one. Also see https://tailspintopics.blogspot.com/2012/01/f-111b-inflight-refueling-probe.html

The third step, also a hand grip, is a peg that extends out of the fuselage just below the bottom of the crew-escape capsule.

To board from the left side after opening the canopy, it was left foot on the bottom step, right foot on the peg on the brace, and left foot on the peg on the fuselage. It would appear that the next move, getting the right-hand leg over the canopy sill, could be challenging for a short person.

The door that housed the bottom step was a little wider at its aft end and a little shorter than the nose landing gear door and centered slightly aft (the aft end of the "ladder" door is located forward of the top of the aft stripe on the landing gear door) along it.

Self-boarding was utilized during the at-sea trials.

According to Jim Rotramel, my F-111 subject-matter expert, most USAF F-111s did not have the retractable "ladder" door: "The only thing common to both variants was the retractable peg in your fourth pic. It served as an anchor for our boarding ladder, fitting into one of two slots, depending on which side the ladder was being used on. It was ‘interesting' to go to a base that didn’t have our boarding ladders, mostly they resorted to step-ladders."

Jim Rotramel

However, it appears that the F-111As were initially equipped with the ladders but they were subsequently removed and follow-on USAF F-111s were delivered without ladders except for the FB-111A. Bill Spidle provided this front view of one at Offult AFB taken by George Cockle on 14 December 1980. Note the angle of the "ladder" door (it was attached by goose-neck hinges on a tangent to the local cross section of the fuselage, which was identical to the F-111B's for the length of the nose landing gear door). What appears to be a widening of the door as it gets closer to the fuselage is actually the inboard side of the brace behind it.

And this FB-111A boarding ladder from the side:

Sunday, December 31, 2023

Friday, December 8, 2023

Sword 1/72 F8U-1P/RF-8 Photo Crusader

22 March 2024: As noted in the original post, an early F8U-1P would require a Vought ejection seat and different wheel hubs than provided in the F8U-1P/RF-8A kit. Modification of the kit seat headrest would probably suffice for the former but exquisite 3D-printed replacement wheels are now available for the latter from Jonathan Smith.

See https://www.ebay.com/itm/186335298796.

Note that the openings in the nose wheel hub should line up.

Smith's main landing gear inboard main landing gear hubs do not include the disc-brake caliper assemblies that are located on the bottom of the landing gear strut but these are easily added.

They are provided on the kit wheels and correctly pictorially oriented if you're paying attention to the instructions:

22 January 2024: This is a work in progress. I'm still making additions and corrections. Please don't hesitate to inform me at tommythomason@sbcglobal of any you have.

This post has benefited significantly from input from Ed Barthelmes and Bill Spidle, who I consider to be Crusader subject-matter experts. For more on this iconic carrier-based airplane, I recommend you add Ed's F-8 Crusader Walk Around Number 38 from squadron/signal publications and Bill's Vought F-8 Crusader from Specialty Press to your library.

Finally, a 1/72 kit of the photo-reconnaissance F8U/F-8 Crusader! And the first impression before building it is excellent (Sword was also provided with pretty good Vought drawings of it). This is Sword's test assembly as an RF-8G"+":


To answer the two most frequently asked questions: there is no option to raise the wing and while some of the detail parts like the nose landing gear resemble that of the Academy F-8E/F-8E(FN), F-8J kit (Tom Weinel's preference: see https://superheatmemorial.blogspot.com/2018/12/172nd-f-8-kit-review.html), it is also clearly different in most particulars. One small flaw that Tom noted in most F8U kits (Heller got it right) that Sword also included: there is no fairing or bulge on the upper wing surface at the wing fold joint on any Crusader.

Pictures of the sprues and decals are here: https://aeroscale.net/news/crusader-box-contents. Note that there is no difference in the kits with respect to the plastic, resin, or even instructions. The only difference is the decal sheet. Additional markings will be forthcoming from Caracal Models. Also, don't lose track of the small rectangular tan piece of paper in the box. Barely perceptible on it are the masks for the canopy, windscreen, camera ports, and the view-finder window.

You may wonder, as I did, about the raised rounded triangles on the top of the inboard leading edge slat;

Bill Spidle informed me that these were associated with the F-8L wing which added pylons, one on each side, to F-8B wings. They were located at the hinges of the leading edge flaps and increased the fatigue life of the wing. After the F-8Ls were retired, their wings still had life remaining to they were utilized for at least some RF-8G upgrades.   They are present on the RF-8G "+" BuNo 146882 on display at the Frontiers of Flight Museum in Dallas, Texas. His photo:

They are clearly not as prominent as the kit's and in any event, are not on the F8U-1P/RF-8A wings.

The F8U-1P prototype (a conversion of F8U-1 BuNo 141363) first flew on 15 December 1955. The last flight of one, an RF-8G"+", was to the National Air and Space Museum on 29 March 1987, over 30 years later. There were numerous detail changes to the configuration over that time. Sword provides most of them in this kit.

There were three basic versions, not counting details like DECM antenna fairings and camera ports:

F8U-1P/RF-8A: The most notable omissions from the kit are that the first F8U-1Ps were delivered with a Vought ejection seat, a nose-wheel hub with spokes, and a fuel vent mast under the left hand side of the aft fuselage. For the seat and nose wheel, see https://tailspintopics.blogspot.com/2009/10/f8u-crusader-variations.html. For more on the landing gear changes, see https://superheatmemorial.blogspot.com/2018/12/f-8-landing-gear.html. The kit only provides a Martin Baker seat that might be either a Mk 5 or a Mk 7. However, in 1/72 scale, these can be distinguished by painting the parachute housing accordingly (see http://thanlont.blogspot.com/2011/02/transition-to-martin-baker-ejection.html). For an introduction to the F8U-1P, click HERE.

RF-8G: 73 RF-8As of the original 144 were remanufactured to be RF-8Gs, including new wings that added a pylon on each side. These were delivered between 1965 and 1970. The most obvious external change was the addition of the ventral fins under the aft fuselage to increase supersonic directional stability (the left fin incorporated the RF-8A's fuel vent mast). For some reason, five USMC RF-8As got the ventral fins with no designation change. The G changes included a beef up the landing gear; the differences might not be readily apparent in 1/72 scale (see the landing gear link above). The tail hook shank went from squarish to round (the kit's looks squarish, i.e. RF-8A).

RF-8G"+": The + is a notation that Tom Weinel added to differentiate it from the G's that were modified to this configuration beginning in 1978. The major external difference was the addition of the cooling scoops on the top of the tail pipe and blocking off one of the small vents on the right side of the fuselage just ahead of the wing. For more on the afterburner differences, see https://superheatmemorial.blogspot.com/2018/12/f8u-engines.html

For more general background, see: https://tailspintopics.blogspot.com/2013/12/photo-gator.html. One comment on it by OldGeezer should be noted when evaluating the usefulness of the following:

From 1975-1977, I was one of the tiny group of engineers responsible for the last 30 or so RF-8Gs at the Naval Air Rework Facility in Norfolk. A lot of things stick in my mind, probably of no interest to anyone these days. You mention the ECM antennas, from memory the last ones I personally saw were Air Frame Changes 598 and 599. I don't think any two of the aircraft that came into our shop ever had identical antenna configurations, and we'd incorporate everything they had missed along with the latest stuff, so theoretically they'd leave our line all with the same equipment. That didn't apply to the cameras though. There were different numbers and locations of windows on the various airplanes, and we couldn't do much about that.

Which DECM antenna configuration you use can only be established by reference to the RF-8 that you are representing.

 The first one on at least a few RF-8A/Gs as early as 1966 is the same as on the Crusader fighter of that era:

It uses part 30. The small circle is a tail light. This was accompanied by a pair of antennas on the bottom of the fuselage between the main landing gear doors that are not provided by the kit.

The second one looks like this:

It uses parts 27, 28, and 30. It appears on Gs in pictures dated 1967 through 1972.

The next one deleted the antenna on the leading edge beginning in 1969. The trailing edge fairing extended farther forward on the fin (the tail light was embedded in the fairing) and there appears to have been two different fairing and antenna configurations. One had multifaceted lumps and appears to have been retained for the remainder of the RF-8's service life:

It resembles kit parts 57/58. I'm not sure when the flare/chaff dispensers were added under the fuselage aft of the main landing gear (they are not provided in the kit, either):

The other was shaped like a bullet and the fairing extended the farthest forward. It may have actually preceded the one described immediately above.

This would be kit parts 65/66.

Finally, an antenna was eventually scabbed onto the right hand underside of the G inlet.

This is kit part 70.

The F8Us did not originally have red anti-collision lights on the top and bottom of the fuselage (they were not a requirement on U.S.civil airplanes before 1957; the military was not required to incorporate them but did). The upper one is provided as a separate clear part. The lower one is molded with the bottom of the camera bay, part CP2, and will have to be removed for the initial F8U-1P configuration.

The photoflash cartridge dispensers, one on each side of the upper fuselage aft of the cockpit, are usually covered by a panel that was removed when required for night missions (for illustrations of previous Navy photo flare dispensers, click HERE).

An F8U-1P with the small-diameter flare dispenser:

An RF-8G with the large-diameter flare dispenser:

Photo via  Ed Barthelmes

The F8U-1P/RF-8A camera system:

Above the light detector window was one of two sensors, either a light monitor for day photos or a flash detector for night photos. The scanner window provided a view for the image motion sensor.

The RF-8G camera system was initially identical to the RF-8A's but eventually diverged. Note the removal of the Station 2 windows from the side and bottom of the fuselage and the addition of a prism window (kit part CP9) at Station 2 on the right side of the bottom of the fuselage. A second prism window was sometimes substituted for the Station 4 window on the bottom of the fuselage. Note that this airplane did not have the Doppler antenna or the single large DECM antenna fairing (see next photo for both) between the main landing gear doors in place of the two in this photo.

This is the bottom of a late RF-8G"+", mainly distinguished by the addition of DECM antenna variations (kit parts 25 and 70) and a Doppler antenna (molded with the bottom of the camera bay, part CP2, so it will have to be removed when modeling earlier aircraft configurations):

I'm not sure why the scanner window appears to be missing in this photo.

The presence or absence of the left or right Station 2 windows on Gs and G"+"s is another mystery. The left window is often missing with or without the prism window present on the bottom of the fuselage. However, the right window is sometimes present even though the left window is not.

Build Notes

There are holes on the inside of the fuselage halves that would need to be drilled out if you are adding the ventral strakes, which are mounted at a 45° angle. The kit parts have different part numbers but I don't know yet if they are actually handed.

Note that the wings were mounted with anhedral of 5 degrees and the horizontal stabilizers, dihedral of 5.4 degrees (n.b. the left and right UHTs were not connected):

Because of the length of service of the photo Crusaders (the high-time one was retired with almost 7,500 hours after 28 years and 11 overhauls) and Sword's reliance on close examination of survivors, there are mid-life and late-life detail additions on the kit parts like reinforcing doublers around the main landing gear wells and a small bulge above and below the stabilizer at its attach point to the fuselage that aren't present on the initial F8U-1Ps.

More to come...

Monday, September 25, 2023

Relying on Museum Examples for Detail Accuracy Part 4

 Actually, I have written about this more than three times. Some unnumbered ones were:

Mk 4 Atomic Bomb: https://tailspintopics.blogspot.com/2011/07/getting-it-right.html

F11F Tiger: https://tailspintopics.blogspot.com/2014/10/f11f-tiger.html

P/F-80 Canopy: https://tailspintopics.blogspot.com/2013/10/lockheed-pf-80.html

A4D-2 Skyhawk: https://tailspintopics.blogspot.com/2012/07/new-airfix-172-4b4p-modeling-notes.html

F4D Skyray Wheels: https://tailspintopics.blogspot.com/2015/07/its-not-that-easy-to-avoid-error.html

J79 Exhaust Nozzles: https://tailspintopics.blogspot.com/2012/12/j79-exhaust-nozzles.html

Grumman F9F-8T Nose Strut Extension: https://tailspintopics.blogspot.com/2014/06/grumman-f9f-8ttf-9j.html

Numbered ones:

Part One (F6U): https://tailspintopics.blogspot.com/2017/06/relying-on-museum-examples-for-detail.html

Part Two (F7U-3M): https://tailspintopics.blogspot.com/2017/08/relying-on-museum-examples-for-detail.html

Part Three (AD Skyraider Vertical Fin: note that in this case, the fin shape of readily available examples has almost always been ignored): https://tailspintopics.blogspot.com/2017/08/relying-on-museum-pieces-for-accuracy.html

This post was inspired by the F11F kit project at DBMK (https://dbmk.co.uk/); also see their Facebook page. Note that their research includes LIDAR scans of an F11F.

Based on their requests and questions, I can vouch for their desire for accuracy, at least in this instance. The latest one was about this feature under the forward fuselage between the forward speed brake and the NACA air inlets.

I'd never noticed it before but quickly tracked this example down to BuNo 141735, now at the Yanks Air Museum at Chino, California. At first, I assumed it was for the attachment of an antenna that wasn't present, probably specific to this particular Tiger since it wasn't evident in pictures of any operational or Blue Angels F11Fs. I finally found one that wasn't BuNo 141735 with a shape there that seemed familiar:


More searching and I found a few more examples of Navy Training Command F-11s with the shape, in particular this one:

That's when the light dawned (no pun intended). At some point after the collision between two airliners over the Grand Canyon in June 1956, the CAA/FAA decreed a requirement for anti-collision lights on U.S. civil aircraft. The military was not required to comply but did so voluntarily. As a result, anti-collision lights were eventually added to the Navy Training Command's surviving F11Fs, including the early ones.

Saturday, June 3, 2023

Early McDonnell F-4B Phantom Configuration

The external appearance of the McDonnell F-4B changed in detail during its time in service. This is not a comprehensive list of the changes to a kit required to reverse it to the configuration when it was first deployed on Enterprise in 1962.

1. Early Bs did not have the slotted stabilizer: http://tailspintopics.blogspot.com/2011/09/f-4-flapstabilizer-change.html

2. Early Bs did not have the bump on the upper surface of the wing over the main landing gear strut or the doubler reinforcement plates on the lower surface of the wing.

3. Early Bs had the Mk 5 ejection seat; the difference in the top of the seat is apparent in any scale: http://thanlont.blogspot.com/2011/02/transition-to-martin-baker-ejection.html

4. The initial external drop tanks on the wing were a McDonnell design; the Navy subsequently procured the cheaper Sergeant Fletcher tanks with a constant-diameter mid section (statements reversing the identification of the source of the tanks are wrong): https://tailspintopics.blogspot.com/2014/02/f4hf-4-370-gallon-external-tank-redux.html

5. The total air sensor wandered around from the nose to the vertical fin and back: https://tailhooktopics.blogspot.com/2022/07/mcdonnell-f4h-total-air-temperature.html 

6. Initially the only external antenna was on the nose landing gear door:

All the other bumps were added later.

7. Deliveries of the IR sensor under the radome were behind schedule and the performance of the system when it was available, a disappointment, so it was usually not present, replaced by a cap. 


When present, the portion immediately behind the dome was probably cylindrical rather than tapered as in the picture above.

 The dome of the AN/AAA-4 sensor is reflective, essentially a mirror.

8. There were detail changes to the cockpit over time but with the exception of the top of the Mk 5 vs. Mk 7 ejection seat in any scale and the instrument panel/radar control in aft cockpit in 1/32, I doubt that they would be discernible by the casual observer. Bill Spidle provided the following illustrations for the Block 22 configuration (circa 1966), BuNo 152244 and subsequent, at least for a while:

Note that the radar scope in the rear cockpit retracted under the instrument panel and the black on either side of the upper part of the instrument panel were curtains (there was also one that blocked out light overhead).

9. One detail missed by most kit designers is that the aft bulkhead of the rear cockpit was never slanted. It was vertical (the compartment was not originally intended to be occupied) and the ejection seat rails were attached to the floor and the top of the aft bulkhead so as to be at the correct angle.

10, The wingtip lights:

For a more complete wingtip discussion, click HERE.

Thursday, February 23, 2023

Douglas A3D Skywarrior Entry/Exit Doors

 The Skywarrior bombers had a self-boarding arrangement* that also did double duty as a bailout slide:

It consisted of an inner (upper) and outer (lower) door. When closed, the inner door formed the bottom of the cockpit floor and sealed the crew compartment for pressurization; the outer door closed off the opening in the bottom of the fuselage. Large indentations, two in the inner door and three in the outer door, served as steps and hand grips and still allowed the doors to have a smooth surface to function as a slide for bailout.

However, the following picture of a crewman ascending into the cockpit included a feature not included in the illustration above, a rectangular transverse ledge (highlighted by question marks) with a raised non-skid pattern.

On close examination (you can see successive ledges below the top one), I finally realized that in this instance, climbing into the airplane had been made easier—particularly if you had something in your hands—by leaning a folding ladder up against the outer door.

* Skywarrior versions had similar arrangement that was different in detail, including the arrangement of steps:

Also see http://tailspintopics.blogspot.com/2013/05/trumpeter-148-a3d-forward-fuselage.html).