Nose Gear Strength

[JeePilot]

How much of a punishment can these take on something lets say the size of a 37 or 319? On landing I've seen some fairly hard nose wheel bounces and was wondering what would consitute a hard enough landing that would either initate a hard landing check afterwords or in the worst case, complete collapse/failure of the gear? Also, are the nose gear tires prone to bursting on exceptionally hard landings? Thanks for your help in advance! :ok:

[JeePilot]

Does anyone have an answer for me?

[exeng]

On jet airliners the main landing gear should arrive first and absorb nearly all of the energy on touchdown; after this the nosegear should have no problem absorbing any energy when it is 'flown' on to the runway. Even if the nosewheel appears to bounce a little I would doubt that this would cause any significant damage. (Remember that the nose gear is built to be tough enough to withstand some fairly aggressive towing forces)

There have been occasions when the aircraft has touched down on the nosewheel before the main gear. (Nearly always as a result of poor handling) In this instance it is very likely that significant damage will occur to the nosegear and the surrounding structure.

Back in the 1970's I remember a very sad looking B.707 sitting in the hanger that had suffered in exactly this way; the bottom of the fuselage nose was only about 6 inches from the hanger floor and the nose leg had been forced up into the surrounding structure. In addition, by the way, a pod had been badly scraped. Perhaps an interview without tea & biccies for all concerned.

I did however once witness a BAE146 arrive at LGW 26L, touchdown nosewheel first, and then continue to 'wheelbarrow' down the runway on the nosegear only for about 300 metres. When we lined up we could see it taxing back down the parallel in what seemed like a fairly normal manner. We asked the tower to pass on what we had seen at the time, but the BAE 146 remained silent.

Hope this helps.


Regards
Exeng

[LEM]

Read this (http://www.dft.gov.uk/stellent/groups/dft_avsafety/documents/page/dft_avsafety_507740.hcsp) interesting accident report.

The derotation design limit for a B757 is 7° per second.
Exceed that and you are gonna break something!

LEM

[NoseGear]

I've been know to take some tremendous punishment from the wife and still manage to roll on!!!!:p :p :E

Nosey:}

[Genghis the Engineer]

The answer for a big aeroplane is that the undercarriage will have been designed to take exactly the loads + safety factors in JAR-25/FAR-25, and to fail at 1 Joule of energy, or 1lb of load more than it absolutely has to take - any company stressman who designs in more strength than absolutely required by the regulations at Airbus or Boeing is in fear of their job. Why?, because the company wants it as light as the regulations will allow it to be.

Posted below is is from FAR-25, it's all a bit legalese but for a big aeroplane the answer normally comes out to something along the lines of "the worst load predicted for normal use, plus 40%".

G




Sec. 25.499 Nose-wheel yaw and steering.

(a) A vertical load factor of 1.0 at the airplane center of gravity, and a side component at the nose wheel ground contact equal to 0.8 of the vertical ground reaction at that point are assumed.
(b) With the airplane assumed to be in static equilibrium with the loads resulting from the use of brakes on one side of the main landing gear, the nose gear, its attaching structure, and the fuselage structure forward of the center of gravity must be designed for the following loads:
(1) A vertical load factor at the center of gravity of 1.0.
(2) A forward acting load at the airplane center of gravity of 0.8 times
the vertical load on one main gear.
(3) Side and vertical loads at the ground contact point on the nose gear
that are required for static equilibrium.
(4) A side load factor at the airplane center of gravity of zero.

(c) If the loads prescribed in paragraph (b) of this section result in a nose gear side load higher than 0.8 times the vertical nose gear load, the design nose gear side load may be limited to 0.8 times the vertical load, with unbalanced yawing moments assumed to be resisted by airplane inertia forces.
(d) For other than the nose gear, its attaching structure, and the forward fuselage structure, the loading conditions are those prescribed in paragraph (b) of this section, except that--

(1) A lower drag reaction may be used if an effective drag force of 0.8 times the vertical reaction cannot be reached under any likely loading condition; and
(2) The forward acting load at the center of gravity need not exceed the maximum drag reaction on one main gear, determined in accordance with Sec. 25.493(b).

(e) With the airplane at design ramp weight, and the nose gear in any steerable position, the combined application of full normal steering torque and vertical force equal to 1.33 times the maximum static reaction on the nose gear must be considered in designing the nose gear, its attaching structure, and the forward fuselage structure.


To this are applied safety factors, thus...

Sec. 25.621 Casting factors.

(a) General. The factors, tests, and inspections specified in paragraphs (b) through (d) of this section must be applied in addition to those necessary to establish foundry quality control. The inspections must meet approved specifications. Paragraphs (c) and (d) of this section apply to any structural castings except castings that are pressure tested as parts of hydraulic or other fluid systems and do not support structural loads.

(b) Bearing stresses and surfaces. The casting factors specified in
paragraphs (c) and (d) of this section--
(1) Need not exceed 1.25 with respect to bearing stresses regardless of the
method of inspection used; and
(2) Need not be used with respect to the bearing surfaces of a part whose bearing factor is larger than the applicable casting factor.

(c) Critical castings. For each casting whose failure would preclude
continued safe flight and landing of the airplane or result in serious injury to occupants, the following apply:

(1) Each critical casting must--
(i) Have a casting factor of not less than 1.25; and
(ii) Receive 100 percent inspection by visual, radiographic, and magnetic
particle or penetrant inspection methods or approved equivalent
nondestructive inspection methods.

(2) For each critical casting with a casting factor less than 1.50, three sample castings must be static tested and shown to meet--
(i) The strength requirements of Sec. 25.305 at an ultimate load
corresponding to a casting factor of 1.25; and
(ii) The deformation requirements of Sec. 25.305 at a load of 1.15 times
the limit load.

(3) Examples of these castings are structural attachment fittings, parts of flight control systems, control surface hinges and balance weight attachments, seat, berth, safety belt, and fuel and oil tank supports and attachments, and cabin pressure valves.

(d) Noncritical castings. For each casting other than those specified in paragraph (c) of this section, the following apply:
(1) Except as provided in paragraphs (d) (2) and (3) of this section, the casting factors and corresponding inspections must meet the following table:

Casting factor Inspection

2.0 or more 100 percent visual.
Less than 2.0 but more than 1.5 100 percent visual, and magnetic particle or
penetrant or equivalent nondestructive
inspection methods.
1.25 through 1.50 100 percent visual, magnetic particle or
penetrant, and radiographic, or approved
equivalent nondestructive inspection
methods.

(2) The percentage of castings inspected by nonvisual methods may be reduced below that specified in paragraph (d)(1) of this section when an approved quality control procedure is established.

(3) For castings procured to a specification that guarantees the mechanical properties of the material in the casting and provides for demonstration of these properties by test of coupons cut from the castings on a sampling basis--
(i) A casting factor of 1.0 may be used; and
(ii) The castings must be inspected as provided in paragraph (d)(1) of this section for casting factors of "1.25 through 1.50" and tested under paragraph (c)(2) of this section.






Sec. 25.623 Bearing factors.

(a) Except as provided in paragraph (b) of this section, each part that has clearance (free fit), and that is subject to pounding or vibration, must have a bearing factor large enough to provide for the effects of normal relative motion.

(b) No bearing factor need be used for a part for which any larger special factor is prescribed.




Sec. 25.625 Fitting factors.

For each fitting (a part or terminal used to join one structural member to another), the following apply:

(a) For each fitting whose strength is not proven by limit and ultimate load tests in which actual stress conditions are simulated in the fitting and surrounding structures, a fitting factor of at least 1.15 must be applied to each part of--

(1) The fitting;
(2) The means of attachment; and
(3) The bearing on the joined members.

(b) No fitting factor need be used--
(1) For joints made under approved practices and based on comprehensive test data (such as continuous joints in metal plating, welded joints, and scarf joints in wood); or
(2) With respect to any bearing surface for which a larger special factor is used.
(c) For each integral fitting, the part must be treated as a fitting up to the point at which the section properties become typical of the member.

[Slick]

what is really freeky is, pulling and pushing the aircraft with a tug attached to the nose gear, awful lot of forces invovled there, I remember with the aero commander you had to attach straps to the main gear, and tow it like that.

[Volume]

This Boeing Aero Magazine (http://www.boeing.com/commercial/aeromagazine/aero_18/touchdowns.html) might be an interesting read.

[LEM]

Didn't know about this Aeromagazine.

Thanks a lot, Volume!

:ok:

[SeldomFixit]

Slick - the towbar is the " weak link " between the NLG and the tractor. Shear pins are rated to take into account breakaway and drawbar forces. Although tractors have been known to punch gears up through electronics bays it would be an extreme and non normal set of events such as brakes parked and excessive force applied by a tractor. I read an article many years ago where a B707 collapsed on a tractor, killing the driver. He was believed to have suffered a heart attack and collapsed in such a way that he applied maximum accelerator with the aircraft brakes set.