By Tommy H. Thomason

Friday, November 16, 2018

Birth of a Legend, McDonnell F4H-1 Phantom II


I've been perfecting my latest and very likely last monograph/book for almost as long as my first, U.S. Naval Air Superiority. At some point, however, you have to either declare victory or surrender if the material is to be shared with those of a similar interest. That time has come for me. Birth of a Legend will be published by Ginter Books (http://www.ginterbooks.com/NAVAL/NF108.htm) and should be shipping in mid-December, just in time for Christmas.

I recommend that you order directly from Ginter Books. It isn't much of any extra cost to you but benefits Steve significantly, enabling him to stay in business, releasing excellent monographs on subjects that the big publishers won't take a chance on. (If you like this one, order my XFL-1 monograph (http://www.ginterbooks.com/NAVAL/NF81.htm); it's pretty good if I do say so myself and he still has lots.)

As the title suggests, Legend is limited, so to speak, to a detailed history of the genesis, design, development, and initial training squadron use of the F4H-1. It is soft-cover, 8 1/2 by 11 inch, and 184 pages (more than 20 in color). It includes at least one picture of each of the first 47 F4H-1s, at least two of which were very hard to come by, as well as a summary history of each one from its first flight to the circumstances of its withdrawal from service. A description of each of the flights that resulted in records and two that tragically didn't is included.

Some of the content is fairly well known but some significant events, like the desk-top evaluation of competing designs at the Bureau of Aeronautics in mid-1954, the redirection of the program from a general-purpose fighter to a fleet-air-defense fighter, the incorporation of boundary-layer control, and the Navy's evaluation/acceptance tests are described in far more depth (and more accurately) than previously. (The fly-off against the Vought F8U-3 was previously covered in detail in Ginter's Naval Fighters No. 87 but is summarized here.)

As is customary in aircraft development programs, changes had to be made as a result of both problem resolution and mission "creep". This is described with numerous illustrations and a configuration summary. A summary of the differences between the 47th F-4A (the redesignation of the first 47 F4H-1/F4H-1Fs) and the 1st F-4B is also provided, with two, the engine inlet and the inflight refueling probe, covered in detail.

Like most Ginter monographs, there is a short modelers section that lists the few kits and conversions that are available for the early Phantom IIs. However, the detail provided in this one will be essential to creating an accurate model of one of the first 47.

However well you know the F-4, I'm sure that you will find information within these pages that you did not know or were misinformed about and pictures that you have not seen before.

Thursday, October 25, 2018

F-4 Flying Under the Golden Gate Bridge

Peter Greengrass, my go-to guy for F4H Phantom stuff, provided some additional information about this incident and another photo.

This picture of an F4H about to fly under the Golden Gate Bridge appears from time to time, rarely with the explanation. The comments usually include statements that it is a fake, imaginative explanations as to why the landing gear is down, the pilot was grounded thereafter forever, etc.

In actual fact, it is not a fake. It was not authorized per se, but also did not result in the pilot making his last flight as an officer in the U.S. Navy. It did involve cameras, often a incentive for a pilot to do something stupid although not in this case. An AirPac-approved camera crew was on board Ranger (CVA-61) to get footage for a David Wolper documentary,  "The Story of a Carrier Pilot". See https://www.youtube.com/watch?v=4MR_bGZrEck

The fact that the footage of this launch is not in the documentary is easily explained by the Navy's unwillingness to appear to condone stupid stunts like flying under bridges, which in any other circumstance would have the pilot's wings removed immediately after landing, assuming that he hadn't screwed up, crashed, and died. The picture above was presumably taken by a member of the public from Vista Point or a boat.

The plan on 19 October 1962 was for XO Ken Stecker  of VF-96, The Fighting Falcons, and another pilot in a second F-4 to be launched well before reaching the bridge after the ship departed NAS Alameda. However, the launch was momentarily delayed (the launch officer was reportedly E. Inman "Hoagy" Carmichael who retired as an admiral, so obviously his career wasn't adversely affected either). When it did occur, Stecker decided that going under the bridge was a better option than trying to climb over it. That was not overly challenging because there is at least 220 feet between the bridge and the water. Stecker subsequently became CO of VF-96.

The other pilot was launched just after the carrier passed under the bridge, as documented by this photo taken from a helicopter. The splash is from the bridle used to launch the jet. It was normally retained (that's what the "plank" protruding ahead of the deck in front of each catapult track was for) and reused, but it was limited to a specific number of launches and it was often simply expended when it was one launch short of the limit if it had lasted that long without incurring visual damage.


In response to one nay-sayer that this explanation can't be true because the ship wouldn't have been going fast enough in San Francisco Bay for the first launch, note that the ship does not have much way on but clearly the wind over deck was adequate. In fact, Ranger didn't have any way on for this dockside launch of a lightly loaded Phantom in Yokosuka Harbor in 1963:


For more on aircraft carrier catapults, see https://thanlont.blogspot.com/2011/01/catapult-innovations.html

Saturday, August 25, 2018

Was the Navy's F-111 Really That Bad?



It was with some trepidation that I awaited the subject article by Robert Bernier in the September 2018 issue of Air&Space Smithsonian. I thought it likely that it would be yet another ill-informed and derogatory recounting of the shortcomings of the Sea P*g. It turned out to be accurate and even-handed in my opinion. There were a couple of things to quibble with at first reading: an editor couldn't resist a snarky subtitle: "Pentagon leaders insisted that an Air Force fighter-bomber would make a great Navy interceptor: They should have asked the Navy"; and I think it's dubious that the side-by-seating was one of the Navy's "demands". However, Bernier also came up with a couple of complimentary F-111B anecdotes that I hadn't read before from individuals with first-hand experience with the airplane, which more than makes up for them and any others.

Bravo Zulu.

Wednesday, July 11, 2018

Grumman TS-2A on Fire

Every once in a while, a picture of a Grumman TS-2A (S2F-1T) with the port-engine propeller feathered and the engine nacelle engulfed in flame gets posted with a request for more information, which is rarely forthcoming.
 Tailhook Association via Doug Siegfried

Here is the story behind the picture:
It appeared in the January 2013 S2F newsletter published by Gordon Bonnel, LCDR USN (Ret), who I'm sad to report has passed on along with the web site that provided back issues...

Friday, July 6, 2018

United States Marine Corps Aircraft Since 1913


The latest aircraft reference book from E.R. Johnson is United States Marine Corps Aircraft Since 1913, published by McFarland. The breadth and depth of its content will be familiar to you if you have seen the author's American Military Training Aircraft: Fixed and Rotary-Wing Trainers Since 1916 (see https://thanlont.blogspot.com/2015/05/american-military-training-aircraft.html), American Military Transport Aircraft since 1925 (see http://thanlont.blogspot.com/2013/04/american-military-transport-aircraft.html), or United States Naval Aviation, 1919-1941 (see http://thanlont.blogspot.com/2011/06/united-states-naval-aviation-1919-1941.html). In this case, it is a 7" x 10" soft-cover 580-page book that is encyclopedic in coverage.

One of the benefits of having this book in your library is that it not only replaces but also updates some classic USMC references such William T. Larking's U.S. Marine Corps Aircraft, 1914-1959. While the only color provided is on the front cover, the reproduction quality of the more than 500 gray-scale photos is more than adequate for a book of this type. Consistent with Johnson's past practice, the photo captions add value, providing useful information.

The text begins with a relatively short history of U.S. Marine Corps aviation followed by three very lengthy sections describing, in turn, Fixed-Wing Tactical Aircraft; Fixed-Wing Transport, Trainer and Utility Aircraft; and Rotary-Wing Aircraft. If Johnson missed describing (specification, summary description, usage, Lloyd S. Jones multi-view drawing) any one of these, it would be very obscure indeed. Coverage is complete up through the Lockheed Martin F-35B and includes the still-borne McDonnell Douglas A-12.

Eight appendices provide summaries of Unmanned Air Systems; Aviation-Related Ships; Aviation Installations; Aircraft Squadrons and Aircraft Assignments,;Aviation Unit Organization; Organizations of Expeditionary and Amphibious Operations; Aircraft Weapons and Tactics; and Aviation Designation, Terms and Abbreviations. A very useful Glossary and Index are included and in the event that the reader wishes to delve more into the subject, there is a very lengthy Bibliography.

USMC Aircraft since 1913 can be ordered from McFarland's website (www.mcfarlandpub.com) or by phone (800-253-2187. It is also available as an ebook from all major ebook providers. See https://mcfarlandbooks.com/customers/how-to-buy-ebooks/ for a list.

Tuesday, May 8, 2018

Strike from the Sea


Specialty Press has formally notified me that they have zero stock and it will not be reprinted. It got pretty good reviews on Amazon. If it's anything like Scooter!, which has been out of stock at Crecy for a while now, the price for one will eventually go through the roof. Fair warning if you have been dilly-dallying about acquiring it for your library.

Crecy, by the way, has expressed interest in a reprint of Scooter! if there is enough new material to warrant it. Most of what I have on hand are error corrections and updates on the civil and contract test/training usage. I need more material on the Israel A-4s. With respect to the latter, does anyone  know of a subject-matter expert?

Wednesday, April 4, 2018

Naval Aviation Centennial Newsletters

While looking for something else, I discovered that the Centennial of Naval Aviation newsletters issued beginning in 2009 to publicize the year-long event in 2011 are still available online:
These contain interesting articles of events both historic and then-current. They can be downloaded at: http://www.public.navy.mil/AIRFOR/centennial/Pages/Multimedia.aspx

North American FJ-1 Carrier Operations

In the process of doing some research for another author on the North American FJ-1 Fury, I had a senior moment that I documented with this post, which I have now corrected. My apologies.

VF-5A was equipped with FJ-1s before NATC finished its evaluation of the type. The urgency to get jets aboard carriers resulted in an unusual clearance direct from the Chief of Naval Operations in February 1948, He issued authorization for the squadron to conduct limited at-sea operations "to expedite carrier operation jet aircraft prior (to the) completion (of) all customary trials." Restrictions included no catapult takeoffs, limited gross weights, no tip tanks, and keeping "vertical impact velocities at a minimum". It advised that at the maximum approved takeoff gross weight of 11,600 lbs, a takeoff run with 35 knots wind-over-deck would be 840 feet on a "hot" day and 680 feet on a standard day (59° F), "based on test results obtained with experienced pilots under optimum conditions". Note that an Essex-class carrier deck was 862 feet long.

On 10 March 1948, with those restrictions, guidance, and permission in hand, VF-5A squadron commander CDR "Pete" Aurand and his executive officer, LCDR Bob Elder, made the first carrier takeoffs and landings by a more or less operational jet squadron from Boxer, CV-21. Life magazine photographers were aboard and aloft to capture the event. Landings were relatively easy, with notably better visibility of the deck than in a prop plane. Deck-run takeoffs were dicey and begun using all the available deck.


During this outing or outings, either the NATC restriction on catapult takeoffs had been lifted or Aurand made a command decision to be catapulted off rather than make deck runs for some of the takeoffs.
 
Based on his and Elder's successful evaluation, Aurand attempted to get all the VF-5A (now VF-51) pilots qualified for carrier operations beginning on 3 May 1948 aboard Princeton. It did not go well. A wing broke off the fourth Fury to land; the wing stayed on deck but the rest of the airplane went over the side (the pilot was rescued). There were too many trips into the barriers, some resulting in damage that precluded continuing without significant repair. The week-long qualification period was reportedly terminated after only two days by the ship's captain. The crippled Furys were craned off at San Diego.

If this was an attempt to claim bragging rights as the first Navy squadron to carrier qualify in jets, it was a failure. On the east coast, VF-17A—flying McDonnell FH-1 Phantoms from the smaller Saipan, CVL-48—was making their first carrier takeoffs and landings, perhaps a few hours before VF-5A began qualification as a squadron but possibly a day or so later. Moreover, VF-17A experienced no incidents, flying as many airplanes off the carrier as they had craned aboard on 1 May and qualified all their pilots. Tragically, the success was marred by a fatal mid-air collision between the commanding officer, CDR Ralph A. Fuoss, and his wing man upon return to NAS Quonset Point. For more, see Squadron Takes Jets to Sea

It is not clear when NATC's formal at-sea FJ-1 carrier-suitability trials were accomplished. I'm all but certain that it was aboard Princeton with these two jets with photo-reference markings. One carried a large "A" on the forward fuselage (BuNo 123066) and the other, a "B" (BuNo 120369).



Based on a note to myself from research several years ago at the Washington Navy Yard, I'm pretty sure that it occurred in August 1948 on the west coast after appropriate beef-ups and modifications were made to the Fury, determined to be required by Aurand's premature attempt to carrier qualify VF-5A. My notes indicate that one of them, "A" , suffered a hard landing with a structural wing failure; however, an excellent picture of BuNo 120366 provided by Rob Eisenberg (his father William was one of the photographers aboard for the trials) indicates that it was stopped by the Davis barrier, which resulted in a nose gear collapse and significant damage to the underside of the nose. The tail hook does not appear to present, which suggests that it may have become detached during the landing. Nevertheless, these trials cleared VF-5A, then designated VF-51, to go back aboard a carrier and qualify as a squadron, which they did in September.

VF-51 provided jet familiarization in squadron strength with their FJ-1s for at least three west-coast carriers before transitioning to the Grumman F9F-3 in 1949.



Sunday, April 1, 2018

Kneeled Deck Parking

Kneeling - it seemed like a good idea at the time. The U.S. Navy's first carrier-based jet, the McDonnell F2D-1/F2H-1 Phantom, had conventional folding wings to maximize the number that could be parked on deck or in the carrier's hangar.
At some point (and this is not an April Fools joke), possibly suggested by Grumman as shown in one of their brochures, the concept of kneeling gained favor:

A "parking dolly" would be inserted under the nose and the nose landing gear retracted so the tail was raised.
Once the pilot had taxied out of the pack while kneeled, the nose gear would be lowered and the parking dolly removed.
The primary benefit was to direct the hot and powerful jet exhaust above airplanes and deck crew. According to the Grumman brochure, there were other benefits as well.
It also made deck parking compact enough that wing folding was not absolutely necessary, reducing structure and system complexity and eliminating the weight, cost, and maintenance burden of that feature.

The Bureau of Aeronautics liked the idea so much that it was a requirement for the next round of Navy carrier-based jets. Ironically, Grumman didn't receive a contract.

The North American FJ-1 Fury's approach was to stick the "kneeling dolly" into a socket in upper strut of the nose landing gear after opening an access door in the forward-facing nose gear door.


 Don Hinton Photos

 I'm not sure what the "guard" was for. It is too flimsy for protection from the arresting gear cables. My guess is that it was to keep the nose wheel from hanging up on something in the wheel well as it pivoted and unpivoted to lay flat. In any event, it appears to have been added after the surviving FJ-1s were assigned to the reserves.

The nose gear was then retracted using a hand pump.
 The Vought F6U Pirate's "Nose Parking Wheel" was inserted into a socket on the bottom of the nose just forward of the nose wheel well.
 Indicative that the primary purpose of kneeling was the redirection of the jet exhaust, kneeling was also required on the McDonnell F2H Banshee procured in the second group of carrier-based jets even though its wings folded, unlike those of the FJ-1 Fury and the F6U Pirate.

Vought considered partially retracting the nose gear to kneel its Model 346A, which became the F7U-1 Cutlass, but decided that since it was tailless, overlapping its nose over the wing of the airplane in front was adequate.

The Banshee's kneeling capability continued in production for a while, as evidenced by this F2H-2N photo.

However, kneeling appears to have rarely, if ever, been utilized operationally and was not required of the next generation of carrier-based jets, including the Grumman F9F Panther.


Monday, February 26, 2018

F-111A vs B - What Drove the Weight?

Another opportunity to set the internet record straight: what drove the F-111's empty weight? Some assume that it was the Navy's carrier-basing requirement. That's not only in question but some of the Air Force requirements penalized the F-111B's empty weight.

It is true that carrier-basing imposes a weight penalty. The FJ-2 Fury weighed approximately 1,000 lbs more than the F-86 on which it was based, resulting it being underpowered with the Sabre's engine. (That was solved by putting an engine with more thrust in the FJ-3.)
However, the major contributors to that penalty are wing folding, high sink-rate landing strength, and tail/catapult hook components and mounting structure. The variable-sweep wing feature required by the Air Force for long-range deployment and high-speed ingress sufficed for wing folding. The Air Force requirement to land on unprepared fields (most runways in Europe were assumed to be cratered in the first day of the war) meant it had a pretty strong landing gear with excellent sink-rate capability. At that time, Air Force airplanes were equipped with tail hooks for emergency landings albeit not to the same strength as a carrier airplane's. Therefore, the F-111A was penalized only by the relatively inconsequential tail hook and nose-tow-launch attach structure (the F-111B's nose landing gear itself was different).

It's not clear which service was responsible for the side-by-side seating arrangement. It is true that the Navy's two-seat jet night fighters, the F3D and the stillborn F6D had side-by-side seating, in part because shortness is a virtue on a carrier and their radar antennas were humongous. However, I've seen no documented evidence that the Navy required side-by-side seating, other than stipulating a maximum length, which the Grumman F-14 accommodated with tandem seating. On the other hand, the Air Force insisted on a heavy and complicated escape capsule that the Navy had no use for. It was best accommodated with side-by-side seating.

The Air Force also insisted on a bomb bay for nuclear stores and its unprepared field requirement (look up California bearing ratio at your leisure) dictated really big, low-pressure tires. Both of those features resulted in big compartments in the airframe that added empty (no pun intended) weight.

However, the biggest Air Force weight penalty was probably imposed by the Mach 1, low-level ingress on a nuclear strike. That results in the need to design for a very high "q" (dynamic pressure) and gust loading of the structure, neither of which were a requirement for a Navy missile-truck loitering on a Combat Air Patrol station at altitude and then dashing off toward an incoming raid. Moreover, given that the Navy's F-111B was not a true fighter, it probably could have been designed for a load factor of 4 rather than 6.5, further saving weight.

It should be noted that one reason for the F-14 being somewhat lighter than the F-111B was that it didn't have any of those Air Force features other than the variable-sweep wing and moreover, the weight of the big Phoenix missiles over and above that of Sparrows was considered an overload from a structural strength standpoint.

Friday, February 16, 2018

One More Time, The Grumman F12F

This is not the Grumman F12F:

It is the Grumman Design 118, proposed to the Navy in December 1955. The Navy rejected it because they didn't want a second development program of a fighter powered by two J79s (the McDonnell F4H was already under contract) but suggested that Grumman go back to the drawing board and propose a single-engine, Sparrow-missile armed fighter to compete with Vought's proposal for a Sparrow-armed "Super" F8U powered by the P&W J75. Grumman did on 4 May 1956.  The Navy rejected it as well in favor of what became the F8U-3 in a letter to Grumman from the Chief of the Bureau of Aeronautics, RADM James S. Russell, dated 16 July 1956, Subject: Grumman Aircraft Engineering Corporation Model 118A Airplane; proposal for: "The recent receipt of more up-to-date engine data does not alter the relative standings of your design with others already programmed in the fighter field. The Chief of the Bureau of Aeronautics had therefore determined that the introduction of another design using the same engines and comforming to the same general operating requirements cannot be justified or undertaken." Neither Grumman or the Navy ever referred to either of these two proposals as the F12F.

There was a Grumman F12F but the designation was assigned to a production variant of the F11F powered by GE's J79. Grumman had proposed its J79-powered Design 98J to the Navy in January 1955 and then its Design 98L, basically the J with increased wing area, in February. The latter appears to be the basis for the purchase order and contract in August 1955 that the Navy created for two F12F prototypes, which were to be assigned BuNos 143401 and 143402. The following artists concept appeared in the Design 98L report dated 15 February 1955.

This is the US Navy's F12F Characteristics Summary dated 15 August 1955. Although there is no drawing, the dimensions and performance data match the Grumman 98L's including the wing area of 350 square feet and the single J79 engine. (The Model 118 and 118A had a wing area of 595 square feet; the former was to be powered by two J79s and the latter, by one J75.)
It is not clear that the contract was ever issued. It probably wasn't. In any event, it was canceled or terminated in January 1956, probably due to the demonstrated performance of the Vought F8U-1 that first flew in March 1955 and the need to fund the development of a competitor to the F4H. However, the Navy had contracted with Grumman in August 1955 to put the J79 in the last two F11Fs in the first production lot in parallel with their plan to buy the F12F. These were designated F11F-1F and were not canceled, since they would provide the Navy with J79 flight experience desired prior to the beginning of the F4H flight test program.

One or more aviation historians have inadvertently conflated the two programs and incorrectly concluded that the Navy contracted with Grumman for its twin-J79-powered Model 118 and designated it F12F. Another enthusiast subsequently speculated that it was given the popular name Lion, which some have accepted as fact as well. It was not.

Saturday, February 10, 2018

The A-12 Avenger II Program - The Fat Lady Finally Sang

I'm embarrassed to say that I neglected to cap this story off when she did back on 24 January 2014. The settlement, according to a Reuters article, was:

"(T)he Navy will receive three EA-18G electronic attack aircraft from Boeing, and a $200 million credit from General Dynamics toward its work on a new DDG-1000 destroyer."

For the Aviation Week report (it may have still been a weekly back then), click here.

For my penultimate post, which has links to prior ones: http://thanlont.blogspot.com/2013/06/the-12-avenger-ii-program-end-is-near.html

(Actually, it turns out that my penultimate post was: https://thanlont.blogspot.com/2014/01/the-12-program-is-history.html)

Wednesday, January 3, 2018

Pre-war Downward-Vision Windows




One of the interesting features I noticed early on as I grew better acquainted with the history and development of U.S. Navy carrier-based airplanes was the presence of downward vision windows on most of the early monoplanes. My first guess was that they were incorporated to somewhat make up for the reduction in downward visibility resulting from the larger wing located more directly under the pilot. An example is the XF2F-1. As in most of the biplane fighters, the pilot sat just aft of the trailing edge of the lower wing.

The most significant benefit from restoring a view directly downward would be the ability to better judge drift from a crosswind, which would provide better accuracy for the dead reckoning necessary to find your way back to a carrier on an otherwise trackless ocean.

And that would appear to be the case for the downward vision windows in the Brewster F2A Buffalo and the Grumman F4F Wildcat.
Note that the F2A window is huge relative to the F4F's (also see Jim Maas' comment below; he is the F2A subject-matter expert), indicative of a lack of specificity for the requirement. The F4F window was not intended to be used to determine whether the landing gear was down, as some have speculated. The pilot can't see the wheels through the window even when the shock struts are fully extended.
The windows are also of no use in landing. A pilot uses his peripheral vision to judge height above, and position over, the runway and the LSO's signals when landing on a carrier.

My original theory, even if correct, doesn't hold for the bombers since the pilot in the biplanes was generally located over the bottom wing. In any event, the earliest monoplane scout bombers like the BT-1 and SBA don't appear to have had downward-vision windows. The follow-on Brewster SB2A, Vought SB2U, and Douglas SBD Dauntless and its replacement, the Curtiss SB2C Helldiver, did. In this case, they were sometimes referred to as Bombing Approach Windows. Note that these were all dive bombers.

This is the Naval Aviation Museum's SB2A when it was conveniently hung from the ceiling. Note the channels on either side of the window for the struts of the bomb-displacement mechanism.

The SB2U's were located immediately aft of the cowl flaps.

The SBD's was located just aft of the post for the bomb-displacement mechanism and is hard to see even in this excellent Miles Lombard photo.
 The SB2C's is even more rarely remarked upon. It was ahead of the bomb bay and aft of the oil cooler flaps on the belly. It was covered by doors, which kept the window clean until needed.
 I have yet to see a picture with these doors open. The SB2C-3/4 pilots manual states that they were "removed from the SB2C-4 and replaced with an access panel". That was the end of the use of bombing approach windows.

The monoplane torpedo bombers beginning with the Douglas TBD Devastator had a downward vision window but in this case, it was for use by the bombardier although the pilot could see downward through it as well. The TBD was originally intended to function as a level bomber as well as a torpedo bomber. In fact, the Norden bomb sight made famous by the Army Air Force was initially a U.S. Navy project. The bombardier would crawl underneath the pilots seat from his seat in the center cockpit to use it.
 Two large doors kept the window clean until needed.

The Grumman TBF was originally designed for level bombing using the Norden bomb sight as well,

In the following picture, the window, located above the number "8636", is covered by a protective flap.

The Vought TBU's bombing window and sight were similarly located aft of the bomb bay as well.

The final variation of the downward-vision window was present in the Vought XF4U-1, Grumman XF5F-1, and Bell XFL-1. In this case it was again a bombing window, but for very specific armament, small antiaircraft bombs housed within compartments in the wings.

The requirement also mentioned another purpose for the window:

As it happened, the antiaircraft-bomb requirement was dropped for F4U production (also see http://thanlont.blogspot.com/2008/06/antiaircraft-bombs.html) but the window remained for a time (according to Dana Bell, "The bombing window was replaced by an access hatch on the 2,531st F4U-1A (BuNo 500066) and the 901st FG-1A (BuNo 13892). I've never been able to find a record of when/if Brewster made the change."