Four point harness for the rear seats of a 170B

[MPKnolle]

Howdy,

I have been trying to find information on upgrading the rear lap belts in my 53 170B to a fixed four point system, but only seem to find SK170-12C3 which is a 3 point system. Has anyone had luck installing a four point to the rear seats? Thank you in advance, Matthew

[hilltop170]

There is not much structure behind the rear seat to attach the 4-point shoulder harness anchors. I think you would have to employ a DER to design and approve the structure or find a sympathetic FAA inspector to approve a design of your own. Anything done back there would involve structural modification which is a sensitive issue.

Later style outside seat belt anchors can be installed that rivet to the inside fuselage skins as on C-185s. There should not be much resistance to that change.

[n2582d]

Matthew,
Check out Fig. 83 in the C-170 IPC. Here's a screenshot of shoulder harness kit

AK 52-3-912D.pdf

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Fig. 83

You'll find two nutplates already in place for shoulder harnesses on the bulkhead at station 90. While the illustration shows a three point harness the nutplates to mount them are roughly centered behind each rear passenger (9" from centerline) so they would work well with a two strap "Y style" shoulder harness. They are reinforced by a "shoulder harness reinforcement strap" -- p/n 0511232-4 (fig. 23-24) which ties those nutplates on station 90 to the station 108 bulkhead as shown below:

Figure 23

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[MPKnolle]

Gary,

Thank you for the pictures and explanation. I was hoping to use the factory attach points and did not know what all the Cessna kit came with besides the belts and mounting hardware. I have seen an offering from B.A.S. at about $450 per seat. I am guessing the higher price covers the STC and reinforcement of the attach points.

[c170b53]

Thanks Gary, you’ve described those straps well. Always wondered, what was their purpose, makes sense.

[n2582d]

Thanks Jim but actually I did a poor job of describing those straps. My recollection was that those straps were something like 0.020" aluminum. I just went out to look at my plane and the straps there are 0.050". (I edited out the incorrect information in my last entry.) At station 90 they are 9" from center which is roughly centered behind each passenger. The straps are each attached at station 108 on my plane with three AN470-4 rivets. That's stronger than I had remembered. In yet another Service Kit for shoulder harnesses, AK170-10, Cessna uses AD470-5 rivets to attach the strap to station 108.

Matthew BAS harnesses are pretty pricey. I'm looking at Wag Aero for my rear seat belts and Y-style shoulder harnesses. I'm also trying to find the part number for the fitting on the shoulder harness that clips into hook p/n 1414025-1. I agree with Richard in that if you want to start modifying the airframe a DER needs to get involved. See FAA guidance here.

[n2582d]

I found the part number for the shoulder harness adjuster that clips into the hook at station 90. It's p/n 443030-401.

443030-401

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The rear harness setup matches what is pictured above for the front seats. As you can see there are two of these fittings on each shoulder harness. This upper one that I've circled in red is subject to AD 2004-19-01. The gist of this fourteen page AD is that the spring on the upper adjuster needs to be removed. See also Cessna's SEB86-08.

So with just a logbook entry (since it's a minor alteration) you can add fitting 443030-401 to Wag Aero's Cessna Y-style shoulder harness and seatbelt. Mount the shoulder harness to hook p/n 0511232-3 or p/n 1414025-1 along with retaining spring p/n 1414025-2 as shown in SK170-12C.

[ghostflyer]

Well this is my second attempt at writing this as I experienced a cyber attack on my computers that I had to shut down to stop any more nonsense .

I would get a DER involved as it could save your life . When there is a inquest to An accident and non standard equipment[ note.. I am not naming Seat belts or other manufactures ] is used , the blame game starts . The Aircraft manufacture is attacked first for the failure To the structure or attachment [the manufacture has the most money] and then the non standard equipment manufacture is then blamed [and some times fairly due to the age Of design and of the changing standards of engineering ] and lastly the installer who also took a short cut in fitting the component . I have had people tell me the strength of a 10/32 screw but only a very small proportion of these are actually tested and the testing schedule is not what happens in real life . [ I have been supplied bolts from a major aircraft manufacture arrive with all the required paper work and has been checked twice by a QC method and when opened had no threads cut [rolled] on the shanks . ] so getting back on the subject of my soap box statement is get a DER to do a drawing and approval . Look at the so called approvals [STC,s] of fitment of seat belts Of alternate manufacturers and then look at what Cessna does these days in their design .
A DER drawing is expensive but how much is your life worth?

To put it bluntly I have NO faith in most of the alternate [PMA] seat belt manufactures attachment procedures. Early in my aviation career I used to investigate aircraft accidents and the number of people that would be still alive if they replaced their seat belts at least every 10 years or on condition or if their seat belts were correctly attached or even worn especially the sash.

[GAHorn]

OK...Let’s review this situation:

There was NO provision or concern for shoulder harnesses at the time our 170s were mfr’d.

ANY harness whatsoever would be an improvement over NO harness.

IF the structure is not altered.... adding a harness made of DUCT TAPE would be an improvement over OEM.

FAA agrees that simple harnesses that do not alter aircraft strucure is an improvement to safety and does not require a lot of expensive engineering costs.

DOH!


A ROPE tied to the back seat lap-blets would improve pilot/copilot survival in an accident....and should not require an aeronautical engineer to install.

Do it!

[MPKnolle]

Gary,

Thank you for the description and part numbers. I am going to run everything by my A&P and hopefully get 53A up inter air again. She has been in the shop getting a few upgrades and repairs and I thought it was a good time to address the seatbelts. While front and back are serviceable and in great condition, I am upgrading the fixed wag-areo 4-point in the front to B.A.S. with the inertial reel and then moving the wag-areos to the rear so the kids are safely strapped in. Thank you again for all your help!

Ghostflyer and Hilltop 170, thank you for y'alls input as well. We will be taking a good look at the attach points to make sure we are comfortable with the strength that Cessna originally provided. So far, all looks to spec, but you can never be too safe.

George, as aways, thank you for the brass tacks answer to keep the conversation grounded.

Once I get these current upgrades/repairs done I will post in the "What have I bought for my 170 lately" section!

Thank you all, stay safe, stay healthy, and enjoy some time in the sky!

Matthew