MolaKule Q&A on Aircraft Structures V

[MolaKule]

Floor panels support seat tracks on commercial aircraft which support passenger seats.

Question 1: Why have seat tracks?

Question 2: How much G force is a seatrack designed to withstand?

 

[Cujet]

All of us know the answer to #1. The seat tracks are configured with 1 inch spacing intervals. This allows very precise seat positioning. If for example the airline notices that the passengers knees are not yet hitting the seat in front, they will move the seats exactly 1 inch closer. This allows for an additional row of seats in the back, which will remain empty.

On a more serious note, the seat tracks are often fastened by innumerable closely spaced 175Ksi screws, to a robust frame that both supports the floor and is directly tied into the aircraft structure. On some aircraft, the floor handles over the wing pressurization duty too. The seat track framework is that strong!

#2 is more difficult. Modern passenger seats are forward crash 16G rated and 9G in the vertical. But the seat tracks must in practical terms, withstand more load than that. The seat tracks, fasteners and framework must also be resistant to corrosion. As corrosive materials and liquids tend to accumulate there. Think spilled Coca Cola or worse. The alloys chosen and the coatings must be really effective and tough.

 

[shortyb]

Seat tracks would allow an airline to adjust seats how they see fit. Some airlines want to pack as much human cargo in the tube as possible, others spread them out a bit. Well, thats my guess.

As far a g-force, at least what is generated during an crash. Don't know an exact figure, but it's got to be a lot.

 

[MolaKule]

Good answers, guys.

I would only add the obvious answer is that seat tracks allow variable seat pitch.

 

[john_pifer]

They frequently have to be replaced on airliners, where, as @Cujet mentioned, they get corroded because of drinks getting spilled and other corrosion catalysts.

And it can be a pretty big job.

 

[Hohn]

Floor panels support seat tracks on commercial aircraft which support passenger seats.

Question 1: Why have seat tracks?

Question 2: How much G force is a seatrack designed to withstand?

I doubt a particular G force is much of a design criterion. Aerospace design is dominated by fatigue considerations because so rarely are you allowed the weight budget for ferrous alloys. Critical fatigue structures like landing gear and so forth obviously get the luxury of steel. But most of the rest of the aircraft is going to be an aluminum alloy combined with (increasingly) composite materials— often a filament-wound carbon composite.

As such, the seats, tracks, and even fasteners are typically aluminum and thus have a fatigue-limited stress level far below whatever the tensile yield or UTS values might be.

IIRC the rule of thumb for aluminum at “room” temperatures is that you aim for a max load <~40% of yield strength. Even still this is a finite fatigue life, but it’s a long enough life that you can somewhat ignore fatigue.

Where elevated temperatures apply, aluminum loses fatigue strength VERY fast. This is why titanium was primarily used in the SR71. It’s not just the small specific strength advantage titanium gives, nor the fact that titanium, like steel, has an “endurance limit” and superior fatigue properties to aluminum, but rather the SR71’s structural operating temperature almost entirely precluded aluminum. At speed, the plane would just come apart unless made so heavy that STEEL was a better option.

Aluminum is a great material for particular applications in aerospace. High temperatures are not one of those applications.

I am not an aerospace engineer, but I did take some aerospace engineering classes as an undergraduate at the US Air Force academy and served in the USAF for 14 years, so I have a bit of exposure to the Aero world.

 

[MolaKule]

"FAA Sec. 23.785 — Seats, berths, litters, safety belts, and shoulder harnesses.​

There must be a seat or berth for each occupant that meets the following:

(a) Each seat/restraint system and the supporting structure must be designed to support occupants weighing at least 215 pounds when subjected to the maximum load factors corresponding to the specified flight and ground load conditions, as defined in the approved operating envelope of the airplane. In addition, these loads must be multiplied by a factor of 1.33 in determining the strength of all fittings and the attachment of—"

Maximum load factors or G forces are determined as defined in the approved operating envelope of the airplane and then the fittings and attachments must be designed to have a load factor multiplier of 1.33.

And then, "Proof of compliance with the static strength requirements of this section for seats and berths approved as part of the type design and for seat and berth installations may be shown by—

(1) Structural analysis, if the structure conforms to conventional airplane types for which existing methods of analysis are known to be reliable;

(2) A combination of structural analysis and static load tests to limit load; or

(3) Static load tests to ultimate loads."

Some slide presentation on same from one of my alma maters:

https://faa.niar.wichita.edu/Portals/0/AC 20-146 Overview.pdf

https://www.fire.tc.faa.gov/2010Con...ivaresCertificationSeats/OlivaresCBAlPres.pdf

In the case of a minor crash landing these loads are defined:

"FAA § 25.561 - General

(a) The airplane, although it may be damaged in emergency landing conditions on land or water, must be designed as prescribed in this section to protect each occupant under those conditions.
(b) The structure must be designed to give each occupant every reasonable chance of escaping serious injury in a minor crash landing when—
(1) Proper use is made of seats, belts, and all other safety design provisions;
(2) The wheels are retracted (where applicable); and
(3) The occupant experiences the following ultimate inertia forces acting separately relative to the surrounding structure:
(i) Upward, 3.0g
(ii) Forward, 9.0g
(iii) Sideward, 3.0g on the airframe; and 4.0g on the seats and their attachments.
(iv) Downward, 6.0g
(v) Rearward, 1.5g..."

 

[BusyLittleShop]

While in the USAF I installed and secure aft facing seats into their tracks on C141 at Norton AFB... Going commercial I always test to see if my seat track is 100% secure by applying my body weight against the back and give it a quick push with my feet...