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PIO and control methods

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PIO and control methods

Old 6th Sep 2004, 00:27
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PIO and control methods

My apologies if this topic has been discussed before...However I noted in another forum that one method to control PIO involved "locking the stick" in position with the knees, at the appropriate pitch attitude, then working the problem from there.

Recently I noted a fatal accident in the ultralight aircraft which was in part due to PIO encountered by an inexperienced pilot. I very much doubt that he knew of PIO, let alone of how to control this flight condition.

Is there a method to control PIO if it occurs, given that I have done some flight testing of homebuilt aircraft in the past? I have always pre-briefed that I would first "reduce the gain" by shifting my grip closer to the base of the stick, then locking the stick once stabilised.

I understand that prevention of the PIO condition (W&B, pre-flight inspections, test planning etc.) is better than the cure...

Nevertheless, is there a universal accepted method of controlling this behaviour if it occurs?

I'd be interested if there are some test pilots out there that have experienced this flight condition, and could relate how they've dealt with it.

Regards

Woxman
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Old 6th Sep 2004, 01:41
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woxman and PIOs

Often referred to as JCs - why - well one of the first to be a victim of a severe PIO was a TP flying an F86 Sabre at high IAS. Sabre was one of first fighters with powered flying controls but it only had spring feel. You could only feel IAS by the amount of stick movement made to produce a result.

It just happened that above about 400 Kts if one flew through a gust disturbance causing the aircraft to pitch a little then the pilot's hand/arm on the stick acts like a bob weight and the pitch of the aircraft is enough to input a pitch demand opposite to what the aircraft is doing. This causes the aircraft to pitch the other way and one rapidly has a runaway condition with the aircraft pitching up and down whilst ever the pilot hangs on to the stick.

Experiencing this unexpectedly caused the hapless F86 TP to yell over an open microphone/radio "Jesus Christ" as a prayer in his moment of need!
Hence the derivation of JC/PIO.

My first was also in an F86 at 500 Kts at 500 ft and I had my left arm down beside the ejection seat. 5/6 large excursion in pitch almost broke my arm as I was projected violently up and down before letting the stick go to break the feedback loop. My next was also in an F86 in a dive on a target at low level attempting to fire 2 30mm cannons with negative g applied. Might have called more than JC but only way to break the sequence then was to haul back hard on the stick.
Next was just after lift off in the first Folland Gnat having a full slab tail plane. It was developmental and a known problem being sorted. My PIO was triggered by the gear retraction and the transient trim change nose up as the nose wheel/speed brake retracted. I instinctively corrected nose down and set the PIO going. Once again broke the feedback loop by a hard pull on the stick but had a few big views of the runway in very short time.

Most PIOs/JCs are divergent and can guarantee a wild ride unless you break the loop quickly.

I would expect that FBW computers have PIO damping. Anyone know?

Was surprised to see mention of PIOs in some helicopter control systems.
Any helicopter pilot care to describe situations where they occur and what you do about it before it beats you into bits and pieces of flying blades.?
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Old 6th Sep 2004, 02:36
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Flying helicopters I've never seen anything so severe, but the correction has been the same.

Reduce pressure on the controls, severity of manoevre, power applied. If that doesn't work/not possible, then a hard pull can help.

I have seen ground resonance in a tandem get out of hand, and then quickly controlled with an arm holding the stick tight and same arm locked hard onto the seat. This tells me that the ground resonance was exacerbated by pilot inputs, so a frozen stick may work.

With an older SAS, I've seen PIO that was most easily controlled by turning off the affected system, although identifying the bad one can be a challenge.

Matthew.
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Old 6th Sep 2004, 03:37
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Thanks Milt,

I'd forgotten about the Sabre. I heard that the aircraft was a handful on conversion. I've seen some old footage showing conversion onto CAC Sabres with the instructor flying chase.
I think it showed a few brief seconds of PIO, with a few encouraging words from the instructor!

Was there every any attempt to correct this deficiency in the Sabre? Or was it simply a tradeoff for better manoeuvre performance elsewhere in the envelope?

Cheers

Woxman
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Old 6th Sep 2004, 06:34
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Woxman

Don't think the pitching oscillations you saw with the Sabre were PIOs. The Sabre had small break out forces - that is you had to push or pull a small force before it would move. This caused beginners to over control each way. Usually mastered after the first flight.

Expect that the PIOs have a relationship with stick force per g which for a Sabre decreased with increase in IAS. That is the stick, having force against a constant spring, became quite sensitive at the higher IASs.

Hard to fix with that system although I would have liked to try a counterweight in the elevator input. Pilots became aware of the hazard and could generally avoid it so like aileron jack stalling at high IMNs they had to grow to accept such little imperfections.

The Gnat was smoothed out by changes to the stick to slab elevator gearing/relationship/ratio.
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Old 6th Sep 2004, 13:38
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Woxman

The very description of the PIO manoeuvre gives the answer to the correct recovery.

A Pilot Induced Oscillation requires the pilot to do things with the controls in order for the PIO to start or continue. This the complete solution is to let go of the controls. If it was a PIO and you let go the aircraft will stop oscillating at once. And I do mean instantly become still.

If it was an oscillation that was fundamentally caused by the aircraft aerodynamics – for example a divergent short period oscillation in pitch on say a short coupled or tailless design - then letting go will not necessarily kill the motion. Clamping the controls might help but only with non powered systems. But be warned clamping is very difficult to do in practice as you will be thrown about by the motion and your whole upper body will be difficult to stabilise.

However I would ask you to rethink your notion of moving your hand down the stick. Gripping any stick lower down means you have to move your hand LESS for a given control deflection than when it is holding the top. This increases the sensitivity of the control system which makes you more prone to PIO.

If your thoughts of moving your hand down were prompted by trying to increase the force needed to deflect the controls then I am afraid experience suggests that any slight benefits from this will be swamped by the bad effects of increased sensitivity.

JF
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Old 6th Sep 2004, 13:53
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John Farley

Thanks for the help on helicopter PIOs.

How bad can they get and is there a cure?

MC
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Old 6th Sep 2004, 18:34
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JF, I totally agree that the action for getting out of a conventional PIO is to let go of the controls. I differentiae between conventional and other PIOs in that I have no experiences of non conventional pilot in the loop problems either with powered, body shift, or electronically interfaced, control systems, but I suspect that there will be some new feature yet to be discovered.

Clamping controls is fraught with problems as you state; my non-powered controls civil aircraft had an interesting control system - fuselage structure coupling via the pilot’s upper body and ‘stiff’ arms. Gripping the column lower down was attempted on one test; at least until the hand wheel struck the pilot in the face!
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Old 6th Sep 2004, 21:46
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Milt - my comments related to fixed wing only. Not sure why you should think they were helicopter related...

It would NOT be on to let go of the stick in many choppers.

JF
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Old 7th Sep 2004, 13:48
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PIOs are a problem in any type of aircraft.
A recent SETP paper at the flight test safety symposium introduced something we probably inuitively knew, but had never been formally stated - the concept of a boundary being a big driver of PIO.
Milt referred to it in the bit about the Gnat on takeoff - the danger of hitting the runway really concentrates the mind and may drive the PIO.
Takes a very good pilot to get out of a PIO at any - proven by the fact that Milt and others talking about this are here to talk about it.
PIO can happen in many different ways- some pretty strong conjecture that the rudder coming off the AA flight in Newark was due to PIO in the rudder- breakout force of 25 lbs, and 35 lbs to get to max deflection.
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Old 14th Sep 2004, 07:13
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PIOs MMIs APCs JCs

Consternation in Flight Test

Some research on the subject shows that we now have 2 more names emerging for PIOs

MMI = Man Machine Interface

APC = Aircraft Pilot Coupling

All meaning the same thing.

Guess someone needs to get a conference going with all the test pilots' schools to unscramble these acronyms/definitions.

Here is a 9 second JC which is certainly a pearler..

I heard an absolute pearler of a software PIO story from a Navy guy recently.
The diesel submarine has FBW engine controls. In this story the sub was trying to nose onto
the ramp at the sub base .
A particular technique to veteran sub drivers was to line up the sub and ramp and then 'burst' the throttle to
give a big momentum push.
Command would be "all ahead full,... pause,... all stop" in about 3 seconds.
Enter problem. The system started ramping up the engine to full power, but would not respond to the "all stop" command UNTIL
full power had been achieved. Engine spool up time was about 9 seconds by which time the sub was motoring at maritime granitous.
Next command was "JC,... all reverse full". Once the system achieved full power it processed the next command, all stop and started winding down by which time impact with the moorings/wharf ectetera had occurred.
The short period of calm was replaced with increasing chaos as the sub spooled up to full reverse power.
The command "all stop" occurred as the tail fins of the rapidly reversing sub split open the side of a tender vessel moored nearby.
With well executed silence our sub captain offered no more commands and removed himself from the loop and eventually both the engine and sub stopped with chaos all around. In this instance the PIO was primary the result of a FCS that processed commands sequentially without a feedback loop based on next commands
(I'm sure a software engineer or FTE would improve on this description).

On the subject of subs I also believe that something similar happened submerged when a command went in to the contril system to 'crash dive'. But it was close to the bottom and before the command could be aborted it had its nose deep in the bottom mud. How embarrassing.
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Old 14th Sep 2004, 13:57
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MMI is a larger subject than just the controls - it's the whole way we interface with the machine.
Aircraft Pilot Coupling was an attempt to be politically correct and try not to blame the pilot for the problem. In most circles, it has been allowed to gracefully fade away to let PIO come back to accepted use.
And don't you just love fly by wire stuff that's done without relation to the real world.
Further to the AA Newark crash aircraft, I was just told by some AA folks that the breakout was 22 lbs, and the force for max deflection was 32 lbs - but the travel for max deflection was just over 2 to 3 inches (or some other ridiculously small distance).
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Old 14th Sep 2004, 14:06
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Careful you don't jump on any bandwaggons, there, Shawn.

I would have said that although it seems highly likely that the Airbus accident was due to PIO, if you are putting in 150lb rudder force stop-to-stop inputs, it doesn't really matter whether the BO+F is 25lbs or 50lbs, you're going to push straight through it.

Don't get me wrong, I don't believe that particular design of directional control stands up very well to scrutiny (even though it was designed for the cruise phase, where rudder inputs have no role-relation). But let's not cloud this issue. I think there are too many people on both sides of the argument who try to make political capital out of accidents.

I would say that in this case the PIO was due to a large delay in roll response (due to roll inertia, normal 'transport type' roll response and the delayed roll response as a secondary effect of beta), and as anyone who has experience of this sort of thing(as I'm sure you do) will tell you, that sort of delayed aircraft response is a real recipe for PIO.

IMHO this accident happened because the pilot was doing what he had been erroneously trained to do - using full rudder displacement to roll the aircraft in a manner which I'm sure makes the subtlety of the directional control system harmony irrelevent.

Just my thoughts on that particular one.
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Old 15th Sep 2004, 14:15
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Tester 07
I realize it's a political hot potato. Regardless of whether the rudder inputs were the right thing or not, the mechanical characteristics of not much difference between breakout and max deflection force, and a limited travel certainly didn't help much.
I remember clearly the day that Ralph Smith (the Smith of the Smith-Geddes handling qualities criteria) burst into my office with a cry of something like 'It was a set up for a PIO' (referring to the AA Newark accident). Ralph was a genius, in the true sense of the word, and his work on eliminating PIOs in aircraft was pretty well regarded within the industry.
We have to teach a module on PIOs to the FAA in their courses here, and the discussions are quite interesting.
I'm always in the market for more videos or stories about PIO, so if you know of any, please forward!
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Old 15th Sep 2004, 14:55
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Shawn, did you look at that AAIB report I posted on the gyroplane longstab thread over on rotorheads?. Some interesting PIO discussion in there, particularly w.r.t control mechanisation.

A Goldwing I used to own (laminar wing, single seat Rutan Vari-eze derived Canard pusher) was very prone to pitch PIO, generally in the few hundred feet after take-off. Not so much an FCMC issue - the BO+F was almost negligible. The aircraft had strong stick fixed (displacement) apparent LSS, but near-neutral stick free (force) apparent LSS. In passing through rotation to initial climb, flying heads out, it was very hard when I first owned the aeroplane to get the position right and a few cycles of ±½g PIO occurred once or twice until I engaged brain and just clamped the stick somewhere about right. After a while I learned to just put the stick in about the right place immediately after rotation and make any climb speed corrections at a safe height where I could afford to go head in. Possibly the huge pitch change with power along with a mismatched propeller that tended to change RPM significantly even with small airspeed changes tended to exacerbate this.

(N.B. Tester 07, if ETPS or any bored TPs fancy such a beast for cheap amusement, I know of an imaculate example with about 40 hours on the clock for sale about 10 miles from BDN - the owner's just sadly died of non-aviation causes but was trying to sell it for about £4k just before he died anyway and I doubt his family want it. Arguably the odd handling characteristics make it an ideal TPS aeroplane.)

G

Last edited by Genghis the Engineer; 15th Sep 2004 at 15:09.
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Old 23rd Sep 2004, 01:03
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Ghengis,

It's interesting to see that you've made reference to gyro longstab. I've seen elsewhere in the web that several accidents have been attributed to PIO in these craft, and that airworthiness authorities are investigating the introduction of guidelines to address the issue.

I don't have the URL, but I suspect that it was the UK CAA?

I have only flown a vehicle of this type once....with an instructor. I didn't see any PIO tendencies at higher airspeed, although I was encouraged to fly with a "loose" grip on the stick.

I would be interesting to get a professional TPs opinion? (I know from the thread that we have some highly qualified ones here!)

Furthermore It would be interesting to see what engineering assessments have been made in this area, noting that this vehicle is mostly relegated to the recreational side of aviation.

Cheers

Wox
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Old 23rd Sep 2004, 06:32
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Relegated to recreational aviation !, Hmmmph, some very dedicated professionals put enormous effort into making recreational aviation safe, as well as cheap. Most of us chose to work in the field. Retires into the corner muttering about the word "relegated".


However, my reference to Gyroplanes was not to imply any general problem there (any more than my reference to the Goldwing was intended to imply a general problem with Canard aircraft). But that AAIB report did make some points about specific design features that tended to induce PIO, and in that context was very interesting.

The AAIB reference, BTW, is at http://www.dft.gov.uk/stellent/group...ty_030918.hcsp and I'd recommend dedicating a few hours to it; it's (in my opinion anyhow) a deep and fascinating piece of work.

The AAIB report referred to longstanding research at the University of Glasgow into the subject, led by an FTE turned academic. Should he, or any of his colleagues, happen to look in, I'd be interested in their views too

G
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Old 23rd Sep 2004, 13:59
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Genghis:
Your comments on the difference between stick fixed and stick free characteristics is very interesting, and only serves to illustrate the need to know and understand these items clearly.
The real question is what is it in the flight control system that would cause the difference to be so marked - and then hence make it PIO prone?
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Old 25th Sep 2004, 05:28
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Shawn Coyle

Look for a growing number of PIO anecdotes/stories amongst SR's emails.

There are a few more pending.

If you consider it worthwhile I could tidy them up and post here.
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Old 27th Sep 2004, 15:47
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Yes please - any stories worthwhile. Also videos / films of same. We have an interesting collection, but always looking for more!
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