CT/4B Crashworthiness Enhancement

CASA DESIGN AND MANUFACTURING SEMINAR 19 MAY 2011

ERIC J. WHITNEY – SENIOR DESIGN ENGINEERAEA PERTH OFFICE

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Presentation Overview

Aircraft description and usage.

ADF crashworthiness requirements.

Aircraft survey and modifications.

Dynamic test article, setup and performance.

Lessons learned and caveats.

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Aircraft Description and Usage

CT/4B is an two seat side-by-side aircraft used for screening and basic training of ADF pilots.

Operated in the Normal and Aerobatic Category, the original aircraft was certified in 1972 to FAR 23 through Amdt 10.

Has gone through various updates to reach the current CT/4E standard (certified in 1992) to FAR 23 Amdt 36 (but exempted from 23.562).

Conventional all-aluminium construction with stick controls. The main spar passes under the pilot seats.

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Design RequirementsADF Requirement to update the aircraft to the current crashworthiness standards.

For the CT/4B this was required to be FAR 23 at the latest amendment. Strategy was to pursue an STC to modify the entire fleet of CT/4 aircraft.

The aircraft already complies with numerous current standards related to crashworthiness, determined by checking the aircraft against current rules; in many cases later amendments have only introduced minor wording changes.

A few areas were identified as requiring an update to modern standards, e.g:• 23.561, 562, 785, 787 – Crashworthiness, crew safety.

• 23.863 and 967 – Flammable fluid fire protection and tank installation.

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23.562 RequirementsWorking together

23.562 is by far the most complex rule when showing compliance:

23.562 (cont’d)The seat/restraint system must restrain the ATD although seat/restraint system components may experience deformation, elongation, displacement, or crushing intended as part of the design.

The attachment between the seat/restraint system and the test fixture must remain intact, although the seat structure may have deformed.

Each shoulder harness strap must remain on the ATD's shoulder during the impact.

The safety belt must remain on the ATD's pelvis during the impact.

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23.562 (cont’d)The results of the dynamic tests must show that the occupant is protected from serious head injury.

When contact with adjacent seats, structure, or other items in the cabin can occur, protection must be provided so that the head impact does not exceed a head injury criteria (HIC) of 1,000…

If dual straps are used for retaining the upper torso, the total strap loads may not exceed 2,000 pounds.

The compression load measured between the pelvis and the lumbar spine of the ATD may not exceed 1,500 pounds.

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Remarks on 23.562The purpose of the 562 requirements is to ensure some of the most critical aspects of a survivable accident are addressed. It is not a panacea.

The 10° test covers the majority of survivable accidents:Checks for adequate head clearance, non-lethal head strikes, non-lethal upper body loads (chest and rib etc), seatbelt strength and performance (pre- and post-impact) and seatback performance.

Typical of a level impact / runway over-run.

The 60° test covers spinal injury and upwards rebound of the occupant:

There is no point surviving the impact if you are disabled / cannot get out.

Typical of a hard landing / stall-in.

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Aircraft Survey

An initial assessment of a number of representative baseline aircraft was made by AEA Perth and Sydney offices.

This important step was required to identify any critical features and to step in prior to dynamic testing.

Head strike envelope checked against US crashworthiness design guide; also checked for injurious objects, etc.

Location of wing spar identified relative to seat, and available downwards stroke determined.

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Head Strike Working together

AMPLE CLEARANCE

Restraint Working together

KEEPER AND CRASH ARCH

HARNESS ANGLE

Aircraft Modifications

Installed OEM rear crash arch as mandatory.

Modified seat:

To increase plasticity and cushion stroke during impact.

Removed some underseat structure to further improve available stroke.

Installed new headrest (critical).

Moved harness keeper upwards.

Installed custom designed cushion.

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Modified AircraftConfiguration #1

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HEADREST AND KEEPER

5-PT AMSAFE BUCKLE

Dynamic TestingIn practice 23.562 mandates full scale dynamic testing.

It is critical that all items be agreed upon, conformed and controlled prior to testing; re-testing is very expensive.

Test article was made from an available timexed production fuselage.

10° pitch and roll seat deformation was not mandated as seat is entirely integral with airframe structure.

10° yaw and 60° pitch rigs were manufactured (it is important that they be heavyweight).

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Dynamic Testing (cont’d)Autoliv facility in Campbellfield Victoria was used.

Hybrid III ATD was used, incorporating Denton 3-axis lumbar load sensor (Fx, Fz, My)

Sled is a ‘full stop’ arrangement with controlled inbound velocity.

A ‘bending bar brake’ stops the sled, by plastically deforming multiple steel bars:

Configuration of these bars requires multiple calibration shots and takes up most of the setup time (‘check pulses’). These are done without the ATD installed.

In all 10 check pulses were run and 6 certification shots.

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Test Rig – 10° YawWorking together

Test Rig – 60° PitchWorking together

Pulse ParticularsWorking together

BAR PICKUP

Dynamic Testing (cont’d)High speed (1000 FPS) footage is always deceptive and it is worth seeing the ‘full speed’ footage to appreciate the actual crash scenario.

This is the 26G crash (in automotive terms, only a 46 km/h impact with a short stop)…

<<< FULL SPEED FOOTAGE OMITTED FROM PRESENTATION AVAILABLE ONLINE>>>

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Dynamic Testing Results10° test using the initial configuration highlighted a fragility in the harness keeper and 5-point buckle.

Head clearance, harness loads and lumbar loads were all otherwise OK.

<<< 1000 FPS SHOT 1 OMITTED >>>

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Dynamic Testing ResultsAfter modifications 10° and 60° tests passed in the 4-point harness configuration.

Head clearance, harness loads and lumbar loads were all OK.

<<< 1000 FPS SHOT 2 AND SHOT 3 OMITTED >>>

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Modified AircraftConfiguration #2

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REVISED KEEPER

REVISED TO 4-PT

Dynamic Testing Results

The 5-point harness was still required by the ADF so we pursued a more robust central buckle and conducted another test in this configuration…

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Modified AircraftConfiguration #3

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ALTERNATE AUTOFLUG BUCKLE

Dynamic Testing Results

<<< 1000 FPS SHOT 6 OMITTED >>>

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Head Path – 10° TestWorking together

PREDICTED

ACTUAL

Head Path – 60° TestWorking together

DOWN STROKE

Lessons Learned and Caveats

Expect the unexpected!

Reading and applying the US crashworthiness design guide (in particular the flail envelopes) will go a long way to addressing most 562 requirements. It is a lot of reading but very worthwhile.

Headrests are almost mandatory. This has been automotive practice for decades. Correctly positioned harness keepers are also very worthwhile.

Even when a seatbelt carries a TSO, it may not be ideal in a particular installation under dynamic loads.

Be careful using brittle plastic buckles (particularly where the plastic is involved in the unlatch mechanism).

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Lessons Learned and Caveats (cont’d)

Passing 23.562 dynamic tests covers a number of potential crash scenarios but does not cover design responsibility in other areas. For example, design aspects not checked by 562 are:

Sharp edges / hard objects to the side of the head.

Sharp edges / hard objects that may impact hands, arms or feet.

Forward fuselage strength (particular to light aircraft): There is no point having an adequate seat / harness if the engine comes into the cockpit and deletes the survivable cockpit volume.

Fuel and vapor issues when fuel is stored in the fuselage.

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