TT Straps on OH58

[Discap]

My buddy has an OH58 that he bought from a local Sheriffs department. I was talking to him about it when the topic of TT straps came up. He says that there is no calendar time limit on his straps.

 

I know that there are several difference between the 58 and a 206, but is he correct on this?

 

Bill

[adam32]

Most likely the difference between a Restricted category (the OH58) compared to Standard (the 206)...

 

As far as I know, all OH-58's are either Restricted or Experimental.

Edited December 28, 2018 by adam32

[iChris]

My buddy has an OH58 that he bought from a local Sheriffs department. I was talking to him about it when the topic of TT straps came up. He says that there is no calendar time limit on his straps.

 

I know that there are several difference between the 58 and a 206, but is he correct on this?

 

Bill

 

Possibly operating on a Special Airworthiness Certificate, Restricted Category, having chosen an inspection program under the OH-58’s original U.S. Army maintenance manual. In that case that would be true.

 

That was the case while the aircraft was with the Sheriff’s Department as a public aircraft. The original OH-58 Tension Torsion Straps were on condition. Nevertheless, most operators with respect to TT-straps, still follow the current Bell limits mandated for the civil 206. However, as a public aircraft, they were not required to even hold a civil airworthiness certificate nor were they required to meet civil aircraft maintenance requirements.

 

The rules change once he bought the aircraft. It’s no longer a public aircraft. In order for him to operate the OH-58 he most obtain or already hold a civil airworthiness certificate for that aircraft. He’s also required to have in place a maintenance & inspection program. He likely chose to follow the OH-58’s original U.S. Army Maintenance Manual were TT-Straps were replaced on condition, as in ( b )below:

 

No person may operate this aircraft unless it is maintained per an inspection program meeting the scope and content described in § 91.409(f). The operator must select and identify in the aircraft maintenance records one of the following programs for the inspection of the aircraft:

 

( a ) For type-certificated aircraft, a current inspection program recommended by the manufacturer; or

 

( b ) For former-military aircraft, an inspection program recommended by the manufacturer or North Atlantic Treaty Organization (NATO) military service; or

 

( c ) An FAA-approved inspection program.

 

In the 1970s three 206B accidents were attributed to a TT strap failure where the component ripped apart under the centrifugal forces of the main rotor. Around 1976 Bell responded by making the straps 1,200-hour life limited components, instead of on condition. Then in 1980, following the fourth failure of a TT strap in a Bell 212 the company added a 24-month retirement life.

 

Bell showed that all four failures were the result of operations in a highly corrosive environment. Company engineers have been working on a solution to the TT strap problem by testing new materials and corrosion-resistant coatings. Customers have complained that the 24-month requirement is too short, increases their operating costs and is not necessary for operators who do not regularly fly near sand and salt water.

 

Nevertheless, Bell Helicopter remains convinced that it is desirable to err on the side of caution, and that considering the factual history it is prudent to maintain the 24-month calendar life retirement. However,tension torsion straps part number 206-310-004 have an airworthiness life of 1200 hours or 36-months.

 

Tension torsion straps 206-010-105-003/-005 will have been removed from service no later than 1 January 1979. Refer to ASB 206-78-1 , dated 14 April 1978 and FAA AD 78-11-02 R1, dated 15 December 1988.

 

Tension torsion straps 206-011-147 and 206-011-154 have airworthiness lives of 1200 hours or 24-months, whichever occurs first. The calendar life of 24 months starts when new straps are installed in a main rotor hub and blade assembly, and are subjected to rotation on the helicopter. Refer to ASB 206-80-9, dated 3 June 1980.

 

Main rotor hub tension torsion straps 206-310-004 have an airworthiness life of 1200 hours or 36-months, whichever occurs first. The calendar life of 36 months starts when new straps are installed in a main rotor hub and blade assembly, and are subjected to rotation on the helicopter.

REF:

Restricted Category Type Certification Order 8110.56B

Airworthiness Certification of Aircraft Order 8130.J

Instructions for Continued Airworthiness (ICA) Order 8110.54A.pdf

 

 

Edited December 31, 2018 by iChris

[Discap]

Chris thanks for the info. He is operating under Experimental Exhibition. Now the question is, “how do you inspect the condition of the straps”?. Surely the blades grips etc. need to come off for a look.

 

Bill

[iChris]

Chris thanks for the info. He is operating under Experimental Exhibition. Now the question is, “how do you inspect the condition of the straps”?. Surely the blades grips etc. need to come off for a look.

 

Bill

 

It is imperative no cracks exist in the exterior surfaces of the TT straps. The Straps external condition and integrity is of the utmost importance. The casing must be air-tight to protect the stainless-steel wiring. Moreover, there’s really no way for the amateur end-user to adequately inspect the TT straps. Only retirement at 1200hrs./24 months or depending on p/n, 1200hrs/36 months. However, for certified maintenance personnel there’s written inspection criteria in Post #7 below.

 

Bell TT strap design evolves enclosing thousands of stainless-steel wires in an air-tight casing. Unfortunately, if moisture and other contaminants get inside the casing through a crack, or if the casing should be poorly made and porous, the stainless steel will suffer unpredictable rates of corrosion.

 

The unsafe condition posed by moisture and other contaminants penetrating the protective casing of a stainless-steel wire-wound TT strap is supported by details from Bell and NTSB investigations. The NTSB found were the TT strap had failed; contamination had entered the TT strap and corrosion had weakened the stainless-steel wires.

 

Lessons learned from fatal crashes due to failed TT straps has led to flight hour limits and calendar lives limits. These limitations are based on the TT strap not suffering from any internal corrosion. All fatigue calculations are out the window if the strength of the stainless-steel is reduced by corrosion.

 

Consequently it is imperative no cracks exist in the exterior surfaces of the TT straps. The Straps external condition and integrity is of the utmost importance. Keep in mind of the centripetal force exerted at the ends of the TT straps is around 7 tons or 14,000 lbs. of tensional stress in addition to the torsional stresses of blade pitch moments.

 

Wires essentially intact

 

Failed in-flight between A & B

 

Failed in-flight

Edited December 31, 2018 by iChris

[superstallion6113]

I've never been a fan of the Bell design wire TT straps. The Boeing tie bars/tt straps used on some of their helicopters that consist of stacked stainless steel plates seems like a significantly more robust and durable design.

[iChris]

“How do you inspect the condition of the straps”?. Surely the blades grips etc. need to come off for a look.

 

Bill

 

For certified maintenance personnel there’s written inspection criteria

 

Bell 206 Component Repair and Overhaul Manual

62-00-00 Page 36-37

 

For OH-58 Models:

Inspect Tension-Torsion Strap Assemblies for defects as prescribed by

U.S. Army TM 55-1520-228-23-1; Para. 5-34a (1) thru (8)

 

Edited December 31, 2018 by iChris

[Discap]

Thanks all. As an update my buddy took this info to heart and has grounded his heli (with 15 year old straps) until they are replaced.

 

Bill

[MileHi480B]

Regarding TT straps, here is something I do not believe I have heard discussed. I am hoping there are those more knowledgeable on FAA procedures than me, who can chime in.

 

I am wondering why someone has not come up with an "Alternate Means of Compliance" (AMCO) for the TT straps. It is an unfortunate reality that all too many perfectly good TT straps are being tossed away due to calendar time - with little flight time - and no real aging. So, why hasn't someone come up with an inspection process to serve as an AMCO.

 

I realize that no one wants to take a chance with unsafe straps, and that is not what I am suggesting. I am simply wondering why - with new digital technologies - we can not come up with an adequate inspection process. Even if we had to send the TT straps off somewhere to go thru heavy spectrometer, Xray or dye-penetrant testing, and even if there was a hefty inspection charge, it would almost certainly be cheaper than replacing them - if not needed!

 

I recall a wing-root/center-section inspection that was required on one of my antique planes. The "AD" almost made owning the plane cost-prohibitive. A doctor friend of mine (who happens to be an AP and a retired Air Force General) use his colonoscope to inspect his plane. It was so effective, he applied to the FAA for an AMOC and got it approved. It literally saved the aircraft from extinction!

 

Certainly the technology exists! Any thoughts? I think it would be an extremely profitable business model, if an inspection center could be set up.