Nomad – Facts on the Fix

It’s not AADR’s intention to cross swords with Australia’s Aviation Writers, all experts in their own field. But this journal feels so strongly about the Nomad and its continuing program, in the interests of both national prestige and employment for GAF employees who have stuck with the project faithfully, that we have dug right back to GAF sources for the REAL story behind these allegations.

A further reason for the ensuing article is to keep before the political decision-makers notice the need for a quick decision on extending the Nomad production line …

At the time of writing, no Canberra decision had been made public about further funding for Nomad production.

AADR’s information at the time of writing is that the Nomad program is nearing its allotted end of run, on current allocations.

Without breaching confidence, we believe GAF is not exhibiting at Farnborough this year – purely because it’s no good taking orders for aircraft GAF might not be able to supply in the future.

All aircraft in the current run have been booked by clients, and there could be a healthy backlog – if only the Federal Government gets its machinery into gear quickly and funds an extended program. AADR believes an announcement by the Government to be both a matter of political expediency and a means of putting GAF employees’ minds at rest regarding continued work.

An AADR team flew to Melbourne to conduct an exclusive interview with GAF executives and technical experts, on what really is the true Nomad modification story.

The following is a narrative version of several interviews held at Fishermens Bend and Avalon:

GAF discovered a problem, resulting in a $1. 6 million recall and modification to Nomad empennages. This all happened before the media unleashed its articles on Nomad troubles, say GAF executives.

The trouble was “white noise” resonance – airflow separation between the tailplane and fin, found out after tufted tests and an accelerometer had been installed in a test aircraft’s tailplane.

A snag where the tailplane tab became worn and unbalanced, leading to possible flutter problems, was another job needing modification.

GAF’s design team said that the airflow separation phenomenon altered some of Nomad’s handling characteristics, but Nomad was by no means unsafe before the mods were installed.

There was no quick way to iron out the Nomad’s airflow separation problem, add Nomad design men. The aircraft’s all moving tailplane meant an added complication to theorising a modification to eliminate vibration – present whenever there are pressure differences between two lifting surfaces.

Complaints of pop rivets working in the tail area were all part of the overall empennage mod program, – vibration elimination.

The first stage was to advise a 50-hour inspection by

operators on the Nomad tailplane area. GAF began a bandaid fix by structurally stiffening up the tailplane tab system – but, as GAF men pointed out, this did not eliminate the cause.

Phase two was immediately put in hand; a series of comprehensive and exhaustive tests using tufted areas and accelerometers in the tailplane.

“These show the vibration levels and the degree of separated flow present around the back end,” explained a GAF design expert.

“When you get separated flow it’s a known fact that it has ‘white noise’ in it from the vibration point of view. So effectively you’re bringing in ‘white noise’ at the control surface, which excites everything in its resonant frequency.

“This, we feel, is what is giving us the wear. We have come up with a fix for this – two fixes that we’ve come up with; first of all, to cure the separated flow, we have placed vortex generators strategically around the affected area.”

(These vortex generators were not, at the time of writing, released to operators in the field; they have been advised to replace pop rivets with solid rivets and carry out a certain amount of lateral member structural stiffening).

GAF took the view that too much wear on the tab system would lead to flutter problems.

“We concede that we have to deal with what could be called backyard maintenance organisations, with this aircraft,” says GAF.

“We have laid down tolerances on backlash which one would have to exceed by almost a factor of two or three before anything of significance occurs in the flutter problem.

“So if operators carry out their recommended 50-hour inspection tests, keep within our tolerances, there’s no suggestion that anything would go wrong …

“But to cater for these backyard organisations, we have gone to mass-balancing the tab as well.”

(Backgrounding this point, GAF men point out that the tolerance was 0. 14in; some operators were found to be working with twice that amount of slack, and weren’t even aware that they had a problem).

GAF expects any rudder tab cracking problems to be ironed out with the elimination of “white noise”.

The problem – which has been the subject of an exhaustive retrofit to operators since last December – This purely one of durability, GAF men emphasise.

Government Aircraft Factories’ retrofit program covers the 85 aircraft in the field at the time of writing. Because Nomad is built in sections, modified tailplanes have been shunted to operators worldwide on a replacement basis.

Seventy-five per cent of Nomads operating have been modified to date. The only limiting time factor is that GAF has put eight tailplanes into the exchange scheme, dictating the time lapse between getting the unmodded empennage back from an operator, installing the mods, and sending the modded tailplane off to the next operator.

Forrestair, Hawker Pacific, and Hughes Aviation in the US have backed up GAF in this $1. 6 million recall program.

(AADR understands unofficially that redesigning the Nomad’s back end is impractical because of financial funding. A complete redesign could also lead to re-certification).

Pilot input from operators flying the modded Nomad indicates that the aircraft is much quieter, especially in the flaps-down configuration. There is no buffeting or vibration sensation, according to pilot reports reaching GAF from the field.

Some pilots claim they can pick the difference, even in cruise phases, say GAF men. They added, with typical. caution, that overall handling is perhaps better – but point out that Nomad had passed a stiff certification program before the latest fix was introduced.

GAF has taken out further insurance; “We’ve been through the N24A program with DoT, and we go through a certain pro forma which is certification to FAR 23 standard. We have finished that, and the firm has agreed in conjunction with DoT, that we’ll do an accelerated wear test on the aircraft.

“We have been authorised to do an extra 50 hours on top of our certification, to check for wear of the tailplane tab.”

(Certification flying has already taken an initial 30 hours; the 50 hours of additional testing will mean a total of 80 hours).

Using a standard commuter-type profile – 50-50 on airports and unprepared surfaces and half-hour flight durations, a 5000ft cruise which normally gives any aircraft the most testing met conditions – the test program was going on apace during AADR’s visit to Fishermens Bend and Avalon.

A fatigue meter has been installed to justify the spectrum.

A total of 50 hours have so far been flown in the test aircraft, modified up to and including Phase Two of the Nomad tailplane update.

“We cannot detect any increase at all in tailplane fatigue,” says GAF’s design team.

To streamline the test program effectively GAF has updated to telemetry.

Accelerometer readings are passed from the test aircraft to ground control, and digitally encoded. At ground control, the readouts are translated back to analog form and fed into a computer for flutter aspect interpretation.

This represents the difference between a multi-speed flight test program spanning up to two and a half months, and real-time translation while the test aircraft is still airborne.

What this means in layman’s terms is that while the test aircraft is climbing back to pre-descent height after a dive at a strictly controlled airspeed, telemetry is being interpreted on the ground giving results of the previous dive.

If there’s no snag on that readout, the test aircraft’s pilot is given the “go” for the next descent …

At the time of writing, the test Nomad was going through its extra check hours without a trace of complaint from the modified tail assembly …

So much for GAF’s modification program. What about that ARDU report?

GAF’s short answer is that the Nomad was tested to Service criteria, not civil usage.

In detail, the nub of the whole GAF argument is that the RAAF at ARDU flew the Nomad in accordance with a Service code, against the code in which the Nomad was certificated. The ARDU pilots apparently carried out their tests in their normal manner, flying tests and reading out characteristics in longitudinal and lateral planes.

They then computed these handling characteristics against the various certification documents – FAR 23, AP 1970 (featuring design certification standards for military aircraft) and other Defence specifications.

“Nomad is certificated against FAR 23 – the civil standard,” point out GAF experts.

“DoT certificated Nomad against FAR 23, adding its own requirements.

“Military standards for a frontline piece of equipment are different altogether …”

The ARDU report concentrated on one aspect of the flight envelope – this was virtually the flaps down, STOL application. Civil operators are not allowed to use the full STOL configuration, according to GAF sources.

Nomad had never been designed to operate to military specifications as flown by ARDU. If the Nomad met those demands, it was coincidental.

The Department of Transport had reaffirmed, even in the light of the RAAF report presentation, that they had no need to stray from any line in terms of altering Nomad certification on the grounds of handling characteristics.

ARDU’s report apparently found Nomad wanting in some instances, when measured up against Military Specs, which GAF men admit as very stringent.

“Service specs are written so that, in the STOL mode for example, an aircraft could be able to fly around all day in this mode,” says a GAF designer.

“As far as general aviation specs are concerned, STOL in the Nomad is just not on for a start … the ARDU reports were very well done, very well instrumented … their comments, as has been pointed out, related to the military specs.”

They criticised some of the handling characteristics of the aircraft, mainly to do with force loadings.

“The controllability of the aircraft – that is to say, the authority of the aircraft – was never in question. In that respect, Nomad exceeded most of their requirements.”

Some of the certification requirements were somewhat odd, GAF added. For example, a typical situation envisaged Nomad at flight idle, descending at somewhere around the 4000ft/min mark, with full sideslip on, maximum rudder and almost holding full control wheel on laterally …

This sort of thing, says GAF, is a little controversial.

In brief, Nomad is not unsafe. It fully meets the civil code requirements.

ARDU, the authority for testing aircraft for Army and RAAF requirements, had tested the second aircraft delivered after the Army had bought it. The Australian Army was now the owner of 11 Nomads, still flying.

AADR’s team was shown the tailplane mods at Avalon.

Unfortunately, time and the accelerated wear test program prohibited an evaluation or demonstration of the Nomad Plus mods.

GAF was later asked to comment on some of the snags mentioned in recent media articles. This is a question-and-answer list:

FLAP EXTENSION IN THE RAIN: “There is a relay in the wing, linked up with the flaps out on the trailing edge, which stops certain things happening. For example, you can’t open the passenger door while the flaps are down …

“What happens is that in rain, the high pressure under the wing forces rain up through the cracks between the ailerons and impinges on the relay. Experiments with a plastic cover don’t really work. We have advised operators to use a silicone sealant on the map for a short distance on either side.”

(Came the story of the operator, walking away from his aircraft, who was amazed to see his Nomad’s flaps extend. Thinking he’d left the Master on, he returned to the aircraft – in time to see the flaps retract. There have been no dangerous situations in flight).

HIGH GROUND OIL TEMPERATURES: Hardly a GAF problem.

“The oil cooler has a separate intake … When there’s any forward speed on the Nomad, there’s oodles of cooling air. On the ground, particularly if you’re taxiing downwind … the fix is to not leave the engine at Ground Idle, but increase its speed a little to get more slipstream back to it … there could be an advantage in increasing the size of the exit slot or putting in an auto-type thermostatically-controlled fan.”

UNDERCARRIAGE POD FLIPPER DOORS MISS SEQUENCE: “We have recommended to certain operators that they take the flipper doors off because they get out of adjustment.

“A small spigot in the mechanism can get out of adjustment and miss the fork in which it fits. We have told some operators that the small amount of drag created by not having the pod flipper door on isn’t really a risk.”

WHEEL CRACKING: “There is no doubt that the standard Cleveland wheels are experiencing cracking problems … we use a standard Cleveland GA wheel … we did not have this problem with the 22s. The 24, however, has a higher all-up weight and uses airline configuration flap settings of only 10 degrees instead of the 20-degree settings on the 22.

“Therefore rotation speeds are higher … the airframe physically rolls along the runway for longer periods, at faster speeds …”

The background to this problem was the higher speeds needed for primary airports. GAF subsequently ordered a completely new wheel from Cleveland, but meanwhile, operators have been alerted to the older wheels’ problems.

An interesting sideline to GAF research into the problem was the fact at least one Australian operator was using nil flap for take-off and only dropping 10 degrees of flap for landing, instead of Manual’s 40 degrees for landing.

Cleveland’s new wheels have been made to GAF’s specific order, add GAF men.

“We think now that we’ve got the new wheels, that’s the end of this particular snag.”

ENGINE COWL CRACKING: Has been a problem, especially in the light of having to hang two stainless steel exhaust stubs on a comparatively light sheet of metal like the lower cowl. GAF has tried several mods, including beefing up the cowl around the oil overflow drain, but no simple fix seems available.

TOP COWLING COMING OFF IN FLIGHT: “There have been four instances of Nomad top cowlings coming off in flight. One operator admitted he did not do up the latches.

“One of the biggest problems is that the oil filler cap is under the top cowl. To get to the cap, the pilot has to undo latches, and very often forgets to do up the SECOND set of latches.

“We’ve put another latch on the thing, but quite frankly if pilots don’t do up latches …

“What we have recommended to some operators is to install a simple light aircraft-type access panel to get to the dipstick.”

WATER LEAKS IN FUSELAGE: “We feel some of the instances happened because we didn’t use enough sealant between the main and rear fuselage matings … instructions have been issued to increase the amount of sealant.”

FLAP MOTORS BURN OUT: “There has been only one instance of flap motors burning out.

“We investigated that problem, and found that the operator, possibly to give his passengers a smoother ride, was in the habit of lowering his flaps ONE degree at a time …

“You can appreciate the increased use of the flap motor.

But the salient point was that, in flap motors, there’s an overrun … and the flap motor is stopped by small brakes. What this operator was doing was wearing out the brake linings, not the motor, by continued use of the flaps in minute stages which threw extra load on the brakes.”