1-218-444-4478 PO BOX 847, BEMIDJI, MN 56619
1-218-444-4478 PO BOX 847, BEMIDJI, MN 56619

Mark on Maintenance – P-51 Landing Gear Retraction Rod End Failures & Safety Spring Installation


Now an AirCorps PMA Approved Part!

Manufacturer North American Aviation
Aircraft P-51 Mustang
Part Number 73-33578-3
Rod End Manufacturer P/N ATE-6N (Schafer)

Proper Description ROD ASSEM – LANDING GEAR RETRACTING STRUT CONNECTING
Location One assembly is located in each main landing gear wheel well
Nickname Gear Links / Landing Gear Rod Ends

AC.A. 01-151-L1, Landing Gear Retraction Rod End Failures & Safety Spring Installation, 15-Nov-2019

P-51 Mark on Maintenance - Landing Gear

Overview / Executive Summary

Nobody wants to witness or participate in a gear up landing in a Mustang. It can easily be a $1,000,000 repair. And while the most common reason a Mustang ends up on its belly is pilot error, another culprit can be the failure of the landing gear rod end. Fortunately, there are preventative measures and fixes that almost eliminate the likelihood of this failure mode. However, even if an aircraft has been modified, it doesn’t totally eliminate the chance of a rod end failure. AirCorps Aviation in Bemidji MN has compiled a comprehensive explanation of the hardware and failure modes of this critical component, with the belief that it is something every pilot and mechanic involved with a Mustang should thoroughly understand. Most importantly if you extend the gear on a Mustang and don’t get a green light, DON’T recycle the gear. Read on to learn why…

 

location

LOCATION

The P-51 Mustang ROD ASSEM – LANDING GEAR RETRACTING STRUT CONNECTING (P/N – 73-33578-3), is located in each landing gear wheel well, between Station 50 and Station 75 of the wing.1

1For purposes of length, we will refer to this assembly as installed on a P-51D per Equipment Installation – Landing Gear Wing  (P/N 106-33014).

MAKEUP

The 73-33578-3 assembly consists of a .625 OD x .120 wall Chrome Moly steel tube that is threaded on both ends to accept two ATE-6N Rod Ends and attaching hardware. Each end is drilled with a clevis pin inserted to secure the rod ends along with a lock washer and nut. It is also vertically drilled and tapped to mount a special bolt that contacts the landing gear down position indicator switch.

ADJUSTMENT OF LENGTH OF ROD END & CLEVIS END IN RETRACT STRUT

FUNCTION

The complete rod assembly component can be simply described as providing two major functions:

  • Provides the connection between the main landing gear strut and main landing gear, along with transferring hydraulic pressure to extend and retract the landing gear.
  • Special bolt 73-54221 contacts the landing gear position indicator switch when in the down position (P/Ns 109-54206 or 109-54226).

CONNECTING PARTS

106-580270 – Strut Assem. Hyd. Landing Gear Operating Complete
to…
67972 – Arm Landing Gear Torque LH (Bendix) – Nicknamed “Porkchop”

AND

106-580270 – Strut Assem. Hyd. Landing Gear Operating Complete
to…
67973 – Arm Landing Gear Torque RH (Bendix) – Nicknamed “Porkchop”

 

ADJUSTMENT OF LENGTH OF ROD END & CLEVIS END IN RETRACT STRUT

Because the hydraulic strut never fully bottoms in either direction, the rod assembly is constantly under pressure or tension at both ends of travel. Because of the hinge-like motion of the cycle, misalignment of the bearings will cause additional pressure and tension, and potentially failure of the rod ends.

function

P-51 LANDING GEAR SYSTEM

The landing gear installation embodies a retractable main gear assembly in each wing panel and a retractable tail gear assembly in the fuselage, each assembly being completely enclosed in the retracted position. All shock struts employ the air-oil combination for cushioning. The positioning of all three landing gear assemblies is controlled simultaneously by one lever on the lower left side of the cockpit.

VARIANTS OF ROD ASSEMBLIES & COMPONENTS

The 73-33578 ROD ASSEM – LANDING GEAR RETRACTING STRUT CONNECTING was originally designed and engineered by North American on April 22, 1941. This design was used on the P-51A, A-36, through the P-51D, including the experimental and lightweight mustang subtypes.

In May 1942 the revised design, P/N 73-33578-3, was drafted for production. This new rod end assembly was indicated by the addition of a the -3 to the part number. Improvements on this rod end assembly from the initial 73-33578 design include the following:

  • The rod end 73-33582 was replaced by ATE-6N manufactured by Schafer Bearing Corp.
  • Addition of lock washers
  • Several minor dimensional adjustments

The original 73-33578 drawing also indicates that the completed assembly is to be finished per army spec FS-21 indicating cadmium plating to prevent corrosion.

INTERCHANGEABILITY

While the 73-33578 and 73-33578-3 are interchangeable, we do not suggest doing so as design improvements were made and prior production lots were deemed inactive for future use.

REMOVAL OF 73-33578 ROD ASSEM – LANDING GEAR RETRACTING STRUT CONNECTING

Use the instructions for landing gear maintenance outlined in Tech Order – T.O. No. 1F-51D-2 (AN 01-60JE-2), Maintenance Instructions for F-51D, F-51M, ZF-51K, and TF-51D, 30-Nov-1956

INSPECTION

This heavily worked and integral component on the P-51 Mustang requires frequent inspection and attention to prevent failure, particularly if operating 70+ year old rod ends.

Some key questions in determining if your 73- 33578 rod assemblies need to be inspected / tested / replaced / repaired:

  • Does your aircraft have lightweight rod ends? (Identification notes below)
  • Have the rod ends and tube recently been inspected?

AirCorps recommends these additional inspections / actions.

  • Rod assemblies should be visually checked during pre-flight to verify there are no visible cracks, tube and rod ends are not bent, and Down Indicator Switch Special Bolt is not bent or rotated from alignment with the switch. Know your aircraft so any change will draw your attention.
  • Replace any lightweight ATE-6N rod ends with the heavier later variants.
  • Magnaflux testing rod ends if performing any landing gear maintenance work.
  • Install a take-up spring as a preventative measure to ensure that, upon a failure, the broken rod does not wedge and prevent extension or retraction (Instructions for installation below)
  • Thoroughly inspect during each annual inspection

As a reference, the Aircraft Inspection & Maintenance Guide – P-51, 00-20A-2-P-51, 7-Nov-1947 outlines a detailed inspection of the landing gear system that should happen during pre-flight, after flight, daily, and at 25, 50, 100 hour inspections.

LIGHT AND HEAVY ATE-6N ROD ENDS

The difference between the often referred to light or heavy rod end can be seen in the transition from the threaded end to the rod end assembly. The early or “light” version of the ATE-6N had a narrower neck in which the diameter reduced in size.

A number of operators have experienced broken rod ends as the result of using lightweight variants. If the rod end on the cylinder end of the rod breaks, the rod will fall and jam on the Sta. 61.5 wing rib. When an extension occurs after the rod breaks the landing gear will extend halfway and stop. With a rod jammed on the rib and a broken rod, the only option will be a gear up landing.

On the right we provide photos to show a lightweight version and a heavy version of the ATE-6N.

FAILURES – LANDING GEAR RETRACT ROD END FAILURE

Broken Landing Gear Retract Rod Ends

How to inspect – visual inspection, magnaflux

Tolerance of damage or wear : Zero

Solutions offered: Replacement of Rod Ends

Failure Location #1
Failure Location #1

The most common break occurs at the neck of ATE-6N or 73-33582 with the majority of failures occurring at the cylinder (inboard) side of the rod assembly.

Failure Location #2
Failure Location #2

In the threads at the check nut where the ATE-6N or 73-33582 meets the steel tube. Again a majority of breaks occur on the cylinder end.

Failure Location #3
Failure Location #3

The least common occurrence we have seen is a blowout of the shell on the ATE-6N or 73-33582. We have only seen this type of failure twice.

Examples of Broken Rod Ends

As the images above show, this end appears to have been rolled off – meaning the force applied to retract the gear has bent the rod end until failure.

RESULTING FAILURE FROM BROKEN ROD END

With the landing gear retracted, a failure at the cylinder end of 73- 33578 Rod Assembly causes the broken rod to fall and wedge on the structural skin of the aircraft and rib at wing station 61.5. If the rod breaks and wedges on the rib, there is nothing a pilot can do to solve the problem in the air.

The rendering below simulates a broken retract rod with the landing gear upper cylinder in the up position indicated in red. The broken rod assembly, separated at the extended cylinder end is colored blue. The majority of the 73-33578 rod assembly will remain attached to the Arm Landing Gear Torque (Nicknamed the Porkchop). The rod assembly will wedge itself between the heavy skin and rib at wing station 61.5 and will prevent the landing gear from fulling extending.

WHY NOT TO RECYCLE THE GEAR

When a failure of either rod end occurs at initial retraction, the pilot will typically observe a brief indication of “Gear Up” followed by a indication of “Gear Unsafe”. This is due to the rod releasing the gear position switch in contact with the plunger located on the retract rod after the rod falls from position. If the rod does not jam, the gear will fully extend and a normal landing can be made. As you will not have a safe indication, cycling the gear may present itself as an option. DO NOT CYCLE THE GEAR, this will not solve the issue and will only open the possibility of further jamming in the landing gear system! The landing gear system relies on timing and position of several components and it is high likely more damage will occur.

When a failure occurs in extension, during landing phase of flight, again the pilot will typically observe a “Gear Extended” indication immediately followed by an unsafe light. Typically if an extended indication followed by unsafe indication is observed the landing gear has extended and locked down. Like the failure in retraction, the rod has become misplaced and will no longer contact the rod position switch. Again, DO NOT CYCLE THE GEAR. Have a spotter observe the extension of the gear.

NEVER CHANGE DIRECTION OF YOUR GEAR WITHOUT COMPLETING A CYCLE

Pilot’s extending the landing gear and not getting a green gear down light should not recycle the gear. In any situation, regardless of the integrity of the landing gear normal or failed, the pilot should never recycle their landing gear without completing a full cycle. The pilot may decide to do this due to an indication or deviation in intent of flight. The system must always fully complete a cycle before a direction change is made.

If you put gear down and then quickly cycle the gear before letting the complete system cycle, you run the risk of putting the door and landing gear timing system out of sequence & possibly preventing extension of the gear. On the down cycle, the gear drop with no timing of the doors. On the upcycle of landing gear both timing and sequencing is occuring.

This out of sequence failure mode has the potential to prevent extension of the gear and can also disturb airflow to the radiator and oil cooler due to open doors disturbing the airflow. Radio call the tower, someone on the ground, or a wing man for confirmation of landing gear position.

This was first take off and gear retraction after aircraft paint job at Sarasota on our way to Sun and Fun maybe 30 years ago.

Wheel wells were not painted but some fumes from stripper or paint got into sequencing valve and expanded o rings. Who da thunk it!

Pilot said gear handle was “stuck” half way up and would not move either direction no matter how hard he pushed. Left landing gear is pinned against clam door under hydraulic pressure. I took picture from my plane while we were thinking about what to do. Not wanting to break gear handle off, we decided to pull T handle to dump hydraulic pressure. Gear handle then went to “down “ easily and gear fell out and three green. Great!

He said that he wanted to “try it again”. Really, really bad idea that we abandoned after some discussion and we flew to Stallion 51 with the gear down and locked. The brothers put it up on jacks and swung gear. Very same thing happened.

Dented clam door a little bit more so stopped doing that.

Replaced a couple bucks worth of o rings and gear worked perfectly. thumbsup. Hammered out dent on lip of clam door and we were on our way to Sun n Fun.

Interesting story and I think with a few lessons.

Always swing gear even when you don’t think it is necessary. Don’t break off gear handle. Landing with gear stuck in position as in picture above would have been a disaster.

If the gear is down and locked and three green, never ever “try it again” in flight. Asking for trouble.

– Tony Buechler – P-51D – 44-72942 “Petie 2nd”

PREVENTATIVE LANDING GEAR ROD ASSEMBLY RETRACT SPRING INSTALLATION

As a preventative measure and safety enhancement, AirCorps recommends the installation of a spring between the retract rod and the extrusion in the gear well on the inboard (cylinder) side of the rod assembly.

Installing Broken Rod End Retract Spring

Parts Required

(2) Spring – MS24586

(2) AN742D10 or MS21919DG10 Adel Clamp

(4) AN42-B4A Eyebolts or AN5261032R8

(4) AN365-1032A Nuts

Work performed between station 50 & station 61.5 of wing assembly

  1. Remove center rivet on back to back stringers (P/N left wing 106-14033-10, -11 & P/N right wing 106-14033-16, -17) – it is important for spring to pull outboard
  2. Install AN42-B4 & AN365-1032A through drilled out rivet on stringer
  3. Attach AN742D10 or MS21919DG10 Adel clamp and AN365-1032A on inboard (cylinder) side of 73-33578 rod assembly
  4. Close Adel clamp with AN42-B4A
  5. Connect spring between eyebolts

WHEN PURCHASING

AirCorps Aviation has PMA approval for 73-33578-3 rod assembly and for the individual parts that make up the assembly.

If you’re replacing any parts 73-33578-3 ROD ASSEM – LANDING GEAR RETRACTING STRUT CONNECTING, ensure airworthiness prior to purchasing by inspecting for cracks, straightness, and finish. If swapping out ATE-6N rod ends, operators and maintainers should confirm them to be the heavier variant.

AirCorps can provide parts, perform inspections, answer questions, and assembly services related to this rod assembly as well as overhaul of landing gear, system components, and installations.

AVAILABLE FOR PURCHASE

ATE-6N – Heavy Rod Ends

73-33578-3-ACP Rod Assembly – Landing Gear Retract Strut Connecting PMA approved

73-33578-2-ACP Tube – Landing Gear Retract Strut Connecting PMA approved

THANK YOU

Thank you to the community of operators, maintainers and mustang experts for assisting and contributing to the composition of this post. Namely Doug Rozendahl, Rich Palmer, Glenn Wegmann, Mark Murphy, Tony Buechler, and NATA for their assistance and support.

DISCLAIMER

WHILE AIRCORPS (VENDOR) BELIEVES THAT THE INFORMATION CONTAINED THEREIN IS ACCURATE AND CORRECT, VENDOR DOES NOT WARRANT THE ACCURACY OR THE CORRECTNESS OF ANY DRAWINGS, MANUALS, OR THE INFORMATION CONTAINED THEREIN. THE INFORMATION, DRAWINGS AND REFERENCE MATERIAL ARE SUPPLIED TO THE CUSTOMER ON AN “AS IS” BASIS WITHOUT ANY WARRANTIES, EXPRESS OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, ANY WARRANTIES OF MERCHANTABILITY OR FITNESS FOR ANY PARTICULAR PURPOSE.  ALL WORK SHOULD BE COMPLETED IN ACCORDANCE WITH FAA REGULATIONS AND APPROVED DATA.

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