Nose gear failure

[Greg Esres]

Technical question regarding gear extension mechanisms:

Our flight school's Seneca had a nosewheel collapse today during
rollout after a landing. The instructor, pilot, and witnesses agree
that all of the airplanes 5 landings were good; none hard and
certainly no nosewheel landings. The lights were all green.

Examination shows that the rod end on the gear extension mechanism was
severely bent and sheared. No other structural damage was visible
inside the gear well. The FAA inspector's first reaction was a
rigging problem, but the Chief Mechanic of the flight school convinced
him that a hard landing had done the damage. There was no indication
when the hard landing might have occurred….could easily have been long
before the one where it collapsed, so no one is blaming the
instructor.

The instructor, who is a big iron A&P, doesn't believe that a hard
landing could do the damage indicated. He thinks that the rod end is
a flimsy piece of metal because it doesn't really bear any load; its
job is just to press against the over-center link to make sure it
stays over center. The load of a hard landing would be borne by other
structures in the assembly and would likely show damage. He suspects
a rigging problem as well. However, since the mechanics making the
inspection are the ones who performed the rigging, they have little
motivation to mention it.

Anyone have any knowledge of these mechanisms that might lend support
to one of these competing theories?

[Aaron Coolidge]

Greg Esres > wrote:
: Technical question regarding gear extension mechanisms:

: Our flight school's Seneca had a nosewheel collapse today during
: rollout after a landing. The instructor, pilot, and witnesses agree
: that all of the airplanes 5 landings were good; none hard and
: certainly no nosewheel landings. The lights were all green.

If you look at the UK's AAIB (Air Accident Investigation Branch) they have
innumerable reports on nose-gear collapse on Seneca airplanes. The general
conclusion is just as you postulate later: the overcenter link didn't.
It seems this is a weak point of the Seneca landing gears. The usual
reason the AAIB comes up with for the gear collapse is "mis-rigged".

You might want to peruse some of their accident reports, they're all
on the web, though I can't recall the address.
--
Aaron Coolidge (N9376J)

[Ben Jackson]

In article >,
Greg Esres > wrote:
>The instructor, who is a big iron A&P, doesn't believe that a hard
>landing could do the damage indicated. He thinks that the rod end is
>a flimsy piece of metal because it doesn't really bear any load; its
>job is just to press against the over-center link to make sure it
>stays over center. The load of a hard landing would be borne by other
>structures in the assembly and would likely show damage.

The whole point of the 'over center' part is that briefly the gear is
actually extended slighty farther than the locked position. Maybe it
was misrigged so that (on that particular landing) the rod only pushed
it to the maximum extension point (which would give you your green light)
then on landing the pushrod would have to bear the whole force as the
gear tried to collapse.

It would probably be hard to see it on jacks. There's no force pushing
against the gear to prevent it from extending (in fact gravity is probably
helping when the plane is on jacks).

--
Ben Jackson
>
http://www.ben.com/

[Greg Esres]

<<You might want to peruse some of their accident reports>>

GREAT suggestion! The UK does a far more thorough investigation than
the FAA does. One accident they examine in great detail is almost
identical to the one that occurred today. I'm going to pass on the
report to our Chief Flight Instructor, just in case he has any
reservations about the instructor involved. Nowhere in the report
does it suggest that a hard landing could be a factor.

Thanks!

[markjen]

I can't answer your specific question, but I can chime in to say that when I
was training in a Seneca, I had a terrible time avoiding slamming the nose
wheel down on virtually every landing. I've never flown an airplane that
was so hard on the nose gear. Unless you carried a great deal of extra
speed into the landing, and immediately lowered the nose after touchdown, it
would slam down VERY hard during the rollout as you tried to hold it off.

Given the kind of abuse the nose gear likely takes, I would strongly suspect
long-term wear/tear and fatigue, rather than mis-rigging. But I have no
specific knowledge of how the Seneca nose gear works.

- Mark

[Greg Esres]

<<then on landing the pushrod would have to bear the whole force as
the gear tried to collapse.>>

That's one possibility the British report suggested, lower downlock
too short.

Looking at the schematics, seems like a hard landing would move the
link to more over-center, putting tension, not compression, on the
rod.

[Greg Esres]

<<I had a terrible time avoiding slamming the nose
wheel down on virtually every landing. >>

Yes, the Seneca is challenging in this respect, which is why this
explanation is likely to find receptive listeners. However, it
appears that a vertical hard landing would likely put tension on the
downlink rod end, rather than compression. And the rod failed under
compression.

Our other Seneca was damaged similarly a few years ago, when an
instructor allowed a student to come in hot, and the airplane
porpoised. However, the gear structure showed a lot of other damage
besides this rod end.

The best technique in the Seneca is to trim very nose up on final, and
use forward pressure to maintain airspeed. With the trim helping you
as the airspeed bleeds off, you can land the Seneca in a full stall
and gently lower the nose. Most don't do that, though.

[Mike Noel]

....or you can use the electric trim during the flair and keep it coming back
until the nose comes down. That makes for much nicer landings in an Archer.
It also makes touch and goes much easier because the aircraft is trimmed for
T.O. instead of a flaps down landing.

--
Regards,
Mike

http://mywebpage.netscape.com/amountainaero/fspic1.html
"Greg Esres" > wrote in message
...
> <<I had a terrible time avoiding slamming the nose
> wheel down on virtually every landing. >>
>
> Yes, the Seneca is challenging in this respect, which is why this
> explanation is likely to find receptive listeners. However, it
> appears that a vertical hard landing would likely put tension on the
> downlink rod end, rather than compression. And the rod failed under
> compression.
>
> Our other Seneca was damaged similarly a few years ago, when an
> instructor allowed a student to come in hot, and the airplane
> porpoised. However, the gear structure showed a lot of other damage
> besides this rod end.
>
> The best technique in the Seneca is to trim very nose up on final, and
> use forward pressure to maintain airspeed. With the trim helping you
> as the airspeed bleeds off, you can land the Seneca in a full stall
> and gently lower the nose. Most don't do that, though.

[Greg Esres]

<<...or you can use the electric trim during the flair and keep it
coming back until the nose comes down.>>

Except ours doesn't work. :-)

I really don't like using trim for this purpose, but it's just
necessary to be able to land this airplane nose high.

[Ben Jackson]

In article >,
Greg Esres > wrote:
>
>I really don't like using trim for this purpose, but it's just
>necessary to be able to land this airplane nose high.

How does that work? Is the tail like the Mooney, where the trim moves
the whole horizontal stab?

--
Ben Jackson
>
http://www.ben.com/

[Greg Esres]

<<How does that work? Is the tail like the Mooney, where the trim
moves the whole horizontal stab?>>

I'm not familiar with the tail of the Mooney. The Seneca does have a
stabilator and an anti-servo tab.

There is no problem with stabilator authority, it's just the amount of
physical strength required is high at low airspeeds.

[Michael]

Greg Esres > wrote
> I'm not familiar with the tail of the Mooney. The Seneca does have a
> stabilator and an anti-servo tab.
>
> There is no problem with stabilator authority, it's just the amount of
> physical strength required is high at low airspeeds.

The Seneca stabilator with anti-servo tab design is pretty much lifted
from the Twin Comanche (no surprise), which is also very heavy in
pitch as it is slowed from blue line to landing speed. Without
massive retrimming, a good landing is impossible. I never did get the
hang of landing mine properly until I got the electric trim working.
Then I would trim all through short final and flare, and it worked out
OK.

Fix the electric trim. The usual problem with inoperative electric
trim in that system is that nobody has cleaned or lubed the trim
pulleys in years.

Michael

[Ben Jackson]

In article >,
Michael > wrote:
>The Seneca stabilator with anti-servo tab design is pretty much lifted
>from the Twin Comanche (no surprise), which is also very heavy in
>pitch as it is slowed from blue line to landing speed. Without
>massive retrimming, a good landing is impossible.

I have a single Comanche. The trim is super sensitive at cruise airspeeds.
There's a very definite reaction to a slight trim change. You could fly
across the country on less than half (maybe a quarter) of the throw of
the crank. In the slow flight regime you need massive trim travel.
Between cruise and landing there are 5 or 6 full turns of trim (not sure
exactly since I use the electric trim). You need to take 2 or 3 of those
off before takeoff or it will be a real adventure.

>hang of landing mine properly until I got the electric trim working.
>Then I would trim all through short final and flare, and it worked out
>OK.

I stop trimming when I hit 80mph, and I'm not sure there's much left, but
you're probably crossing the fence faster than that in a twin. Even so
it's a lot of work on a go-around while you wait for the trim to come out.

>Fix the electric trim. The usual problem with inoperative electric
>trim in that system is that nobody has cleaned or lubed the trim
>pulleys in years.

In my case the trim motor (behind the baggage compartment near the top
of the plane) had 3 wires going to it, and the shortest one was the only
strain relief, and it eventually broke off.

Be careful not to get any lube on the trim cable or the pully surfaces,
though. If you do you'll never get it off and the motor won't have any
traction.

--
Ben Jackson
>
http://www.ben.com/