Retractable Undercarriage
 

Chaps,

Just looking for a bit of insight from those of us that fly smaller aircraft with retractable undercarriage, such as the A109.

Do the nose wheels simply castor? If so is the castoring degree of freedom generally lockable for start-up and shut down, or the machines rely on the rear wheel brakes to prevent fuselage yaw on the ground during start-up?

Many thanks,
CRAN

Yes, generally castoring, self centering when no weight on wheels, and lockable for start/stop and rolling T/O & landings.

Many thanks.

CRAN

AS365 is self centring and lockable for running landings / take offs but isn't locked for start up/shutdown.

The self centring uses two cams which engage as the oleo extends after take off - if the nosewheel isn't centred, the gear won't come up due to a microswitch.

The 109 has strict limits on nosewheel angle, towing can be tricky in a tight hangar when you hit the limits. But the S76 has a fully castoring nosewheel, and it is possible to have it 180 out before retraction - so pilots need to be careful to taxy forward a bit to ensure it is pointing the right way before retraction.

Pumas in the early days had cases when the nosewheel would stick at 90 degrees before it retracted. You soon knew about it when a towing lug punched a hole in the nosewheel bay roof.

Getting to the OP, the wheel brakes should also be set to hold the helicopter against torque during start and stop even with the nose wheel lock engaged. The static friction of the nosewheel tyre is not always enough to hold against a spirited application of torque.

:-) I love your choice of words there John

Reminded me of a time in Bombay when one of our now departed pilots used a slightly less diplomatic form when the other pilot was starting the 61 sans rotor brake and she started rockin an rollin..... "Pour the Pi$$ to her ole pal" LOL

Let me guess Rocky R.?

Thanks for all the posts guys; really useful as always.

Other than the obvious reduction in roll-over stability (tripod versus quad), the extra weight and the lack of ability to land on soft or unprepared surfaces are there any other undesirable characteristics of retractable wheels?

Thanks in advance?
CRAN

It sounds like you are aware of landing on soft surfaces, where the wheels sink down and the hydraulic lines to the brakes get torn off.

Worse still the as365 which sunk one side to the stabaliser... Pilot carried on and later in flight this tore off causing the aircraft to tear apart mid air

http://9m-igb.mot.gov.my/download/Ai...dentReport.pdf

Like when doing slam acceleration maintenance checks John. ;)

"are there any other undesirable characteristics of retractable wheels"

Well there's the cost of purchase and maintenance.

Retractable wheels are going to work normally off the hydraulic system (although may be electric). During landings at ad-hoc sites especially, they are the most vulnerable part of the system to damage, and therefore can compromise the rest of your hydraulic system unless fail-safe protections are built in.
The possibility of the undercarriage getting stuck (or partially deployed) is obviously another undesirable characteristic)

The most obvious may be that people forget to lower them for landing!

Maintenance costs
Increased complexity of hydraulic system.

Good guess......but I suppose the ole pal part gave it way ;-)

Retractable undercarriages are fitted to helicopters for the same reason that birds fold their legs; it reduces drag. As a example when the S76A first came out there were occasions were there would be and undemanded undercarriage lowering. When it happened to me the speed dropped from 145 to 135 knots immediately. The 330 and the 332, when required to operate with the gear down would also suffer from speed and range penalties.

Skids are a waste of time when moving large passenger loads as they found out with the 214ST. There was pandemonium when it was positioning on the apron and it was soon fitted with a wheeled undercarriage, albeit, not retractable.

The problems arising with retractable undercarriages do not manifest themselves very often. Certainly not enough to worry anybody.

There was an occasion on the snowy slopes of Norway where a Gazelle fitted with skids landed an shut down. The pilot had just left the aircraft when it slid down the slope and impaled itself on a snow bank. The calls went out and the pilot's superior came out, landed on, shut down and proceeded to lambast the first pilot. At this the second Gazelle slid down the slope and mated with the first one.

We did not have the same trouble with the Pumas; slam it in hard, the wheel would break through the crust and it would rest on its belly. An the end of the day when the weather was closing in and the squaddies were retreating down the hill to be picked up the German 205s had to give up.

We, however, could nail our Pumas to the side of the hill.

What was the result - quintuplets?Never heard of it happening, what was the cause? Electrical presumably, gear handle, relay etc? Ex many hours on A.

Some electrical gizmo upset by the VTA.

It was incredible that some nut case at Bell decided to put an aircraft as heavy as the 214 ST on skids, Alan Bristow took one look at it and declared them insane. We did however have one A/C in from a third party, that had a rear skid brake as it was being pushed out of the hanger, the resultant bang could be heard throughout the building, and brought everybody down from all floors.
It was very lucky that no one was underneath at the time.

Dave, was Ringo´s foot not involved hence an issue of safety boots all handlers?

I was told by an older feller that in olden Daze, in the first year after introduction of the type, that if the 76 was a fixed wing, every one of them in the company would have been landed gear up due to gear extension problems. Don't know if that is true or not.
In the early 2000s we had a 76 that we flew with the gear pinned down for over a week while waiting for spare parts. Even flying with the draggy (if that is a real word) gear doors open we only seemed to loose about 4-5 knots with the same power settings.
Always made me wonder if a fixed gear with fairings would not have been just as efficient, but not as pretty, .also a considerable weight saving by getting rid of all that gear extending/retracting hardware plus a much simpler hydraulic system. I assume I am being simplistic.
Please understand I have a "bit of time" in the 76 and think it is a fine aircraft. Never had a gear problem with it.

Sevarg
Yes I can confirm that Ringos foot was injured in that accident,for which I felt guilty, as I had asked him to help us get the thing out of the hanger, just as he was on his way home. It was not serious thankfully and he was soon back enjoying a pint.

albatross, conversely we always used the S76 gear down as 'airbrakes' on ILS approaching the MM. It was guaranteed to wipe 10-15kias off the speed and bring us back nicely onto spec; we kept 140 up until then as for Essendon 26 we often had Tullamarine jet traffic bemoaning a helicopter being 'in their way'. Bomber Brown put one 727 driver back in his box when he established that the S76 was doing 145 and the 727 a mighty 135 ;)

Only troubles I recall with the 76 was occasionally we had trouble getting the gear to lock up, which a few recycles usually fixed. Problem was proximity switches needing attention. Had one interesting one where the nose gear refused to extend on reaching home. Had a pile of sandbags put down and landed with the nose resting on same. Oleo had not extended on the previous take off and scissor link jammed up against the actuator.

On the smaller side (OP question) there was the Augusta A109A II which failed to extend its landing gear at Redhill, with David Cameron on board.


Press "sensational" report and AAIB report; the handle came off which eliminated both the main system and the back-up system.

I have occasionally been directed to "make best airspeed" in the A109 when being vectored in for an ILS, to fit in with the jet fixed wing pattern. 165kts on final approach is often enough to prompt ATC to ask me to slow down a bit because I'm catching up the preceding airliner. One has to do these things to show them we rotary folk can be very flexible. ;-)

But not lowering the gear above 130 kias I assume......

The added complexity of the hyd system can be a problem. In the early days of the Puma in the RAF I lowered the gear at the end of a night sortie. The pressure in the nose jack blew the end off leaving me with only the two mains down. Raising the emergency gear lever stopped the remaining hyd fluid dumping, so at least I got the AP back. Oh, and I was low on fuel, so had to do a refuel in the hover while sandbags were obtained to build a mound for the nose area during landing.

Oldbeefer, and if the nose leg "down" hose blows, even the "emergency reserve" of hydraulic fluid goes through the same hole....as I found out in Belize. The cavitating hydraulic pump makes a very frightening noise, too.... :-(

The Puma, and subsequent descendants, had a restrictor valve inserted in the nose jack at the down hydraulic entry to slow the lowering and avoiding it slamming against the internal stops. I had it happen once but I didn't worry about it after I had isolated the hydraulics. On landing the nosewheel is jammed against the bay wall and the debris from the fractured jack prevents it from moving back so sandbags are unnecessary.

This was in the Thetford PTA and it was a toss up as to whether to fly it back to Odiham as both hydraulic tanks were still full. In the end we changed the nosewheel jack on site.

One additional question guys:

How does the parking brake typically function? Is it controlled solely at the pilots discretion or does it automatically release or come on when the gear is lowered?

Cheers,
CRAN

At pilots discretion in all that I've operated: I can't think of any reason for it to be automatically applied!

Some aircraft use auxiliary hydraulic pressure for the parking brake, which will bleed off and thus render the parking brake useless for the next start. Tedious as you should then chock the mainwheels when leaving the machine and especially before the next start to secure the aircraft. You then have to remove the chocks before taxi which can require groundcrew to remove and (possibly) stow them back in the boot of the helicopter before you taxi off on your merry way.

It functions by the pilot (usually) pulling out the park brake handle and pressing the footbrakes hard, then (sometimes) turning the park brake handle - before start, so he can then put the chox in the boot.

To release, usually just press the footbrakes (handle pops in) or press the brakes and turn the park brake handle.

It is possible to get airborne from a hover and forget to release the park brake, which then comes as a rude surprise when you do your next running (screech!) landing.

On the A109 series pulling a T handle applies the parking brakes on the main wheels via the utilities hydraulic system, so no need to apply pressure to the toe brakes. Turning the same handle locks them on. But the pressure dissipates after an indeterminate time after shutdown so the aircraft needs to be chocked.

Thanks for the replies guys, most useful.

With regards the toes brakes, do they brake each rear wheel independently as they do on tractors so that you can readily do spots turns? I presume this is the case...

Many thanks,
CRAN

Yes; they do.

Thanks 212man.

CRAN