Chris Lord October 31, 2018

[Kolibri]

As I tried to clarify, "graveyard spiral" was just my rough visual metaphor for what the witness described (going round and round, and descending).
It's reassuring, however, that gyros are not prone to the phenomenon.

________

In my experience when a gyroplane is trimmed properly; it needs no rotor control input to fly straight and level at whatever airspeed is desired.

Well, yes, Vance, that's what trim is for.
But the scenario I'm musing about assumes not only compromised pitch authority, but possibly trim as well.

I take off without trim in a Cavalon and it does not nose dive (rotor dumped forward?) and flies with very little cyclic back pressure.

At something near cruise AS, I can see how rotor blowback effect can negate the need for trim (at that particular AS).

But N198LT was not flying anywhere near cruise AS in its last 20 seconds.
In fact, witnesses reported very little forward speed. Winds were light. Thus, AS was low.

We all know that slow flight requires considerable back cyclic (or nose-up trim) in comparison to cruise.
If control authority were compromised at a low AS, it would present a real challenge.
Trim may not provide enough compensation.

I don't know the strength and length of arm of the Cavalon pitch trim cylinder rod.
Would it be sufficient to, for example, pull back the torque tube for a vertical descent? Or is it less powerful than that?
I'm trying to explore the limits of what Cavalon pitch trim could do in the event of loss of cyclic.

_______
loftus, thanks for your description of that personal incident.
I'm wondering if something like that happened onboard N198LT.


_______
It would helpful if pictures of the Cavalon control system were posted, especially of the lower area.

Regards,
Kolibri

 

[Vance]

As I tried to clarify, "graveyard spiral" was just my rough visual metaphor for what the witness described (going round and round, and descending).
It's reassuring, however, that gyros are not prone to the phenomenon.

________

Well, yes, Vance, that's what trim is for.
But the scenario I'm musing about assumes not only compromised pitch authority, but possibly trim as well.

We all know that slow flight requires considerable back cyclic (or nose-up trim) in comparison to cruise.
If control authority were compromised at a low AS, it would present a real challenge.
Trim may not provide enough compensation.

I don't know the strength and length of arm of the Cavalon pitch trim cylinder rod.
Would it be sufficient to, for example, pull back the torque tube for a vertical descent? Or is it less powerful than that?
I'm trying to explore the limits of what Cavalon pitch trim could do in the event of loss of cyclic.[/COLOR]

_______
loftus, thanks for your description of that personal incident.
I'm wondering if something like that happened onboard N198LT.


_______
It would helpful if pictures of the Cavalon control system were posted, especially of the lower area.

Regards,
Kolibri

That is why I mentioned the pneumatic trim was at the top where the FAA confirmed the controls were in place.

The Cavalons I have flown could be trimmed for a vertical descent.

Somehow Chris was able to go from near VNE to slow flight.

Pictures of nearly every part of the Cavalon are available on line from AutoGyro GMBH.

 

[Kolibri]

Pictures of nearly every part of the Cavalon are available on line from AutoGyro GMBH.

Not that I've been able to so far find. Please post some links, thanks.

That is why I mentioned the pneumatic trim was at the top where the FAA confirmed the controls were in place.

I'm not talking about the pitch trim arm connecting to the torque tube (and rotor brake).
I've been wondering about something in the cabin.

Question: In "Brake" mode, is the pitch trim cylinder pressurized to force the rod up, thus moving the rotorhead forward and activating the rotor brake?
Or, does "Brake" mode completely depressurize the pitch trim cylinder, and the rotorhead falls forward all on its own?

Regards,
Kolibri

 

[Vance]

My recollection is neither.

There is an air cylinder that pushes the rotor head forward and a rotor brake cylinder that are activated in the brake mode.

When the switch is made from brake to flight it dumps the pressure in the system.

There is a different cylinder that pulls the head back for trim in the flight mode when pressure is added to the system.

It has been more than a year since the last time I flew a Cavalon so hopefully someone else will confirm or correct my recollection.

I don't care for pneumatic trim systems.

 

[Alan_Cheatham]

My recollection is neither.

There is an air cylinder that pushes the rotor head forward and a rotor brake cylinder that are activated in the brake mode.

When the switch is made from brake to flight it dumps the pressure in the system.

There is a different cylinder that pulls the head back for trim in the flight mode when pressure is added to the system.

It has been more than a year since the last time I flew a Cavalon so hopefully someone else will confirm or correct my recollection.

I don't care for pneumatic trim systems.

There is a pneumatic cylinder that trims the roll axis and a larger cylinder that provides for pitch trim, which also engages the rotor brake and pushes the head forward in brake mode.

Details can be found in the Cavalon Maintenance Manual which can be downloaded here:

https://www.manualslib.com/manual/1198918/Auto-Gyro-Cavalon.html [h=1][/h]

 

[Tyger]

From the manual:
"Trimming is effected by varying trim pressure in the pneumatic trim actuator which is installed in parallel with the rotor head tilt for pitch control.
Aft or nose-up trimming activates the electrical compressor and increases trim pressure, causing the actuator to contract, and tilting the rotor disc aft.
Forward trimming opens the pressure relief valve to reduce trim pressure and allows the rotor disc to flatten, due to the spindle head offset and the gyroplane's weight."

Sounds like a loss of pressure would put things into full forward trim.

 

[Kolibri]

I researched my above question, and found the answer in an AutoGyro maintenance manual.
When switched to "BRAKE" the pneumatic trim is pressurized, but with reverse effect (my emphasis is in bold):

63-51-00 Rotor Brake System

The rotor brake system consists of a brake pad mounted to a bracket which is hinged to the rotor head
bridge. With the pneumatic mode selector in BRAKE position the operation of the pneumatic trim
actuator is reversed so that increased pressure causes the actuator to push the rotor head up (or
level) and presses a brake pad against the rotor head disc. In order to increase brake pressure,
move the 4-way trim switch to aft. Note that this action will also push the control stick forward. At full brake
pressure the control stick will be maintained in its full forward position.

IF . . . the pneumatic mode selector in N198LT somehow failed and went into "BRAKE" mode in flight,
then Chris would have involuntarily lost altitude from a flattening rotor disk.

More alarming (and confusing) would have been decaying rotor rpm from the engaged rotor brake.

This may explain the odd power fluctuations heard in the Mayday call.
Chris may have been trying to maintain altitude and rotor rpm via the throttle.

If the rotor brake was engaged in flight, he could have lost enough rrpm to explain the otherwise inexplicable plummet from 150' AGL.

Those who may be skeptical of my "trim runaway" theory may be interested to learn that AutoGyro's
latest Cavalon POH is version 3.1, announced on 5 November 2018 (several days after the crash).

SECTION 3 - EMERGENCY PROCEDURES has some new material from the previous version 2.2 (my emphasis is in bold):

SECTION 3 - EMERGENCY PROCEDURES
3.8.4 Trim runaway

Failure of a trim selector switch or pneumatic valve may result in trim runaway (where the
trim system runs to one extreme and pushes the control stick accordingly). Although the
average pilot is able to resist the out-of-trim stick force and continue to fly the aircraft it may
be possible to reduce the stick load by intervention:

(i) High forward stick load required to prevent aircraft nose rising (this will be
coincident with a high air-pressure reading) – briefly turn the Flight/Brake selector
to “Brake” to deplete system air pressure. If the air compressor is heard to start
and the pressure rises again then pull the circuit-breaker marked “Comp” to stop
the compressor. Repeat the brief selection of “Brake” to deplete system air
pressure as required.

(ii) High aft stick load required to prevent aircraft diving (this will be coincident with
low or zero air pressure) – check “Comp” circuit breaker, if activated push to reset
then try to trim aircraft nose-up. If unsuccessful then continue to expedited
landing. Note: reset the circuit-breaker once only.

Not a word was mentioned in scenario (ii) about a possible pressurization of the pitch trim cylinder in reverse, pushing up the actuator arm.

Also new in this Cavlon POH version 3.1 SECTION 3 - EMERGENCY PROCEDURES is:

3.9 Pitch oscillation recovery
3.10 Vibration

All three of these items may have been relevant for N198LT.
I find this very interesting.

Regards,
Kolibri

 

[loftus]

Very interesting!!!

 

[Vance]

The Cavalons I have flown dump the pressure when operating the brake to flight switch because it is porting the pressure in the opposite direction.

The nose down trim button on the cyclic also reduces the pressure.

If the trim pump is running away it is easy to pull the breaker to stop the pump. The pump will stop by itself when a given pressure is reached.

With no pressure the trim system doesn't do anything and it is easy to fly the aircraft. Most of my initial takeoffs were done with the trim pressure at zero.

When a Cavalon is trimmed incorrectly it is easy to overcome the trim and fly the aircraft.

I have not tried to fly a Cavalon in the brake mode. I suspect it would not be a problem.

There is an overdrive button that relieves the pressure in the rotor brake that allows you to reposition the rotor if you are not happy with where it stops.

 

[Kolibri]

If the trim pump is running away it is easy to pull the breaker to stop the pump.

If one remembers to do so during an in-flight emergency.

The pump will stop by itself when a given pressure is reached.

If all systems are functioning properly.

I have not tried to fly a Cavalon in the brake mode. I suspect it would not be a problem.

I have not flown a Cavalon in the brake mode. I suspect it would not be a problem.

Perhaps you're not sufficiently thinking through the scenario.
The affected pilot would have to muscle against what could be an effectively locked up pitch actuator.

Also, the AutoGyro rotor brake pad has a lot of surface area, and the new rotorheads have a double pad.
Their braking power seems ample.

Why do you "suspect it would not be a problem" to fly a Cavalon in "BRAKE" mode?

Regards,
Kolibri

 

[Vance]

Why do you[/COLOR] "suspect it would not be a problem" to fly a Cavalon in "BRAKE" mode?

Regards,
Kolibri

Because the trim force is not very hard to overcome.

 

[Kolibri]

Are you not assuming that the "BRAKE" actuator-up force is similar/equal to the "FLIGHT" actuator-down force?
And that a malfunctioning system would not add additional force than normal?

 

[Vance]

Are you not assuming that the "BRAKE" actuator-up force is similar/equal to the "FLIGHT" actuator-down force?
And that a malfunctioning system would not add additional force than normal?

Yes I am imagining the force would be the same in both directions because I have felt the cyclic with the switch in both "flight" and "brake" positions.

I haven't flown a Cavalon with it in "brake" mode.

I had a client pump the Cavalon to maximum pressure for takeoff by accident and it was easy for me to fly the aircraft.

 

[loftus]

I do tend to agree here with Vance, a malfunction might create difficulty but should be flyable, especially by an experienced pilot like Chris. Everything considered so far I'm still thinking a control system failure.
It could be so valuable if Autogyro would chime in with anything they know. Autogyro owners, particularly Cavalon and Calidus, but also MTO would really like to know I'm sure. I'm sure liability issues make it difficult for them to do. MTO has a similar pneumatic brake and trim system to the other two, except no roll trim in the older MTO. Would be interesting to know how a runaway pump would occur all of a sudden in flight. It would be very difficult to accidentally change the control from Flight to Brake unintentionally. Thank you for the interesting discussion. I keep thinking of Chris,and what catastrophic failure could have occurred that was beyond his abilities.

 

[Kolibri]

because I have felt the cyclic with the switch in both "flight" and "brake" positions.

When it was functioning normally . . .

I haven't flown a Cavalon with it in "brake" mode.

Perhaps somebody would be brave enough to try it and report their experience?


__________

It could be so valuable if Autogyro would chime in with anything they know. Autogyro owners, particularly Cavalon and Calidus, but also MTO would really like to know I'm sure. I'm sure liability issues make it difficult for them to do.

loftus, they've not even announced an In Memorium that their USA COO died in one of their products.
I doubt they'll "chime in with anything they know".

But they sure were busy revising the Cavalon POH at about the same time.

 

[loftus]

loftus, they've not even announced an In Memorium that their USA COO died in one of their products.
I doubt they'll "chime in with anything they know".

But they sure were busy revising the Cavalon POH at about the same time.

Yeah agreed but not surprised. At the same time I know Autogyro have tended to stay off the forum because as you know it can become a shitshow.
I'd still like to know the mechanism of a runaway pump etc. Clearly electrical, just don't understand quite how it happens.

 

[Vance]

It is not hard for me to imagine one of the little wires to the buttons on the cyclic shorting out or the buttons themselves sticking.

In my opinion the brake to flight switch seems pretty bullet proof with the most likely failure being to lose pressure altogether.

If the detent failed and it switched from "flight" to "brake" in flight it would be disquieting rather than catastrophic.

It is hard for me to imagine enough force generated by the trim system for it to be a problem for an experienced pilot.

 

[Kolibri]

I'm wondering what max air pressure would be if the limiting valve/device failed and the compressor never cycled off.
That pitch trim cylinder is quite large, and reportedly quite powerful.



8 December 2018___________________

The Calidus, MTOsport, and MTOfree POHs do not have the new Emergency Procedure items of Trim runaway, Pitch oscillation recovery, and Vibration.
Seems odd since they also use the pneumatic trim system with FLIGHT/BRAKE switch, and would require such text in their POH.

However, the MTOsport 2017 (POH-M17_1.1_EN.pdf) does have them.
AutoGyro_POH_MTOsport-Model2017 Revision 1.1 – Issue Date 13.04.2018

What's interesting, however, about the Cavalon POH 3.1 is although it does have the same verbiage, it hasn't the MTOsport 2017 POH's careful layout.
The text looks very sloppily copied/pasted. Also, the Cavalon POH 3.1 in German has the better layout, however, the concurrent English version seems rushed.
This reinforces my hunch that it was done consequently to Chris's crash, and hurriedly so.


___________

If the detent failed and it switched from "flight" to "brake" in flight it would be disquieting rather than catastrophic.

Perhaps some curious yet brave AutoGyro owner could take off from a long runway, maintain 5' AGL and try it.

Nevertheless, what I'm postulating is a much different scenario than the Cavalon POH's 3.8.4(ii) where the compressor popped a breaker and the pitch trim cylinder had little/no pressure.
Rather, not only is there "BRAKE" pressure on the up-actuator, but it also refuses to dissipate.

I've recently carefully inspected an AutoGyro rotorhead, and talked to some folks.
When the "BRAKE" mode is active, the actuator arm is pressurized to slide up, and moves the pair of hinged levers flush with
the underside of the inner torque tube to hinge up the rotor brake. These levers are about 35mm long, providing significant leverage.
As it pushes up the rotor brake, it simultaneously pushes the rotorhead forward to the stop.

If Chris added nose-up trim, in "BRAKE" mode (accidental or failed) this would have been backwards and moved the actuator further up.





If that pitch trim actuator arm had been pressurized up, even if overcame by muscled aft-cyclic the actuator would still be braking the rotor.
Furthermore, as I understand the mechanism, increasing aft-cyclic would also increase the braking force by pulling the rear of the torque tube into the actuator. (This was not described in any AutoGyro POH.)

Again, I don't know if the pneumatic selector (or switch) failed and locked up the actuator, but it would explain a lot.
Such a malfunction would have presented a very confusing in-flight situation, with little time to diagnose it.

I think it a bad idea to combine trim and rotor braking under one switch and valve. IMO, they should be separate controls and systems.
As Einstein once quipped, "Everything should be made as simple as possible, but not simpler."


____________
N198LT dropped out of the air from 150' AGL.
That has never been adequately explained.

The only thing that seems to make sense of this NTSB witness statement:

In an interview and a written statement, a witness stated he was driving southbound on the highway that paralleled the shoreline of the lake at the time of the accident.
He said the gyrocopter was travelling northwest bound, about 300 feet above ground level "with very little airspeed" and appeared to be turning to the east.
The gyrocopter then "entered an autorotation" then, when it reached "… about 150 feet the nose of the aircraft dropped immediately turning toward [the] east then back toward the north."
The gyrocopter descended from view [and crashed].

. . . is decayed rotor rpm.

And what so far seems to me the most plausible reason for decayed rotor rpm is that the rotor brake was engaged.

Chris seemed to be ruddering around in those last moments.

when it reached "… about 150 feet the nose of the aircraft dropped immediately turning toward [the] east then back toward the north."

This portion may be telling.
An engaged rotor brake would cause the Cavalon to yaw left, so he may have been fighting that with right rudder.
If the rotor rpm had decayed sufficiently, and the Cavalon began to fall, he may have then had too much right rudder, which swung the nose east.
As he quickly corrected, the nose swung left back north.
I think during those very last seconds, all he had was power and rudder, and neither one could help him by then.

 

[Doug Riley]

Sometimes "stupid simple" is not so stupid.

For comparison: The original Wunderlich rotor brake was quite fail-safe. It was/is engaged by manually pushing a plastic puck up against the forward edge of the rotor head. It would only stay there (and stay engaged) as long as full forward stick held it there. Back off stick pressure, and it simply dropped down and disengaged. If worse came to worst, the casting that held the actual brake pad pivoted on the forward end of the torque bar on two rather thin ears of cast pot metal. If engaged hard, the ears it would just break off, again disabling the brake.

 

[Kolibri]

I think it was Einstein who quipped, "Things should be made as simple as possible, but not simpler."