Fire-by-Wire F-35A Gunfire Testing

47th Annual International Symposium 08 – 11 August, 2016, Wichita, KS Fire-by-Wire
F-35A Gunfire Testing
Major Charles M. Trickey
Major Douglas Rosenstock
USAF, SETP Member
USMC, SETP Member
William J. Norton
Douglas Fielden
Lockheed Martin, USAF Retired, SFTE Fellow
USAF
1.0 ABSTRACT
The Joint Strike Fighter (JSF) conventional take-off and landing (CTOL) F-35A incorporates a
25mm gun under a ‘canoe’ fairing above the port intake. This supports the air-to-ground mission
with an expected three firing passes and a total 181 rounds. The low-observable installation has
a port door normally concealing the barrels and a purge door that permits ram air to facilitate
gun gas evacuation from the gun bay. Fundamentally, ground and flight test were intended to
demonstrate gun system compliance with specifications and so requirements, especially
defining any high-altitude/low-speed ‘pinch point’ where insufficient airflow through the bay
could exist to evacuate explosive gases safely for ‘stutter’ firing. The test planning from the
earliest date adopted a new approach. This began with a risk reduction measurement of gun
bay ventilation during non-firing flights to inform the predictive Computational Fluid Dynamics
(CFD) analysis of the gun bay internal flow. This was followed by further measurements and
gun gas sampling during ground gunfire that also verified proper gun system operation and
suitable structural support. A suction fan provided the predicted minimum essential airflow
through the bay. The gas samples were used to quantify the constituent elements. This
permitted airborne gunfire with multiple firing events per flight without gas sampling provided the
airflow instrumentation indicated suitable ventilation. Past programs had captured gas samples
in flight with only one shot event per sortie. There was no room inside the F-35A for sample
bottles and the program sought greater test productivity. Apart from validating this new
approach to gunfire testing, the eventful testing provided other lessons learned from which
future programs will benefit.
SOCIETY OF FLIGHT TEST ENGINEERS
Copyright 2016, Lockheed Martin Corporation. All rights reserved.
Approved for public release, AER201604025 JSF16-505.
Page 1
47th Annual International Symposium 08 – 11 August, 2016, Wichita, KS 2.0 ACCRONYMS AND ABBREVIATIONS
AFTC
CFD
CTOL
ITF
JSF
g
GD
GSCU
HOTAS
IPP
LLAHS
LM
MFD
mod
MS
NAS
NGC
Nz
P&W
PGU
psf
q
RSO
SDD
sec
SMS
TM
TP
VMC
Air Force Test Center
Computation Fluid Dynamics
conventional take-off and landing
Integrated Test Force
Joint Strike Fighter
normal load factor (Nz)
General Dynamics
Gun System Control Unit
Hands-on Stick-and-Throttle
Integrated Power Package
Linear Linkless Ammunition Handling System
Lockheed Martin
Multi-Function Display
modification
Mission Systems
Naval Air Station
Northrop Grumman Corporation
normal load factor (g)
Pratt & Whitney
Projective Gun Unit
pounds per square foot
dynamic pressure
Range Safety Officer
System Design and Development
second, seconds
Stores Management System
telemetry
Target Practice
Vehicle Management Computer
3.0 BACKGROUND
Prior to this testing, the gun had never been fired installed in the F-35A. All testing was
performed during 2015 and 2016 at the Air Force Test Center (AFTC), Edwards Air Force Base
in California, by the JSF Integrated Test Force (ITF) with support from Lockheed Martin (LM)
design and manufacturing teams in Fort Worth, Texas, and vendor support from Northrop
Grumman Corporation (NGC), General Dynamics (GD), Pratt & Whitney (P&W), and BAE
Systems. Ground gunfire occurred at the gun butt (Gun Harmonization Range) site at Edwards.
Airborne gunfire was performed at China Lake Naval Air Station (NAS) and the sea test range
complex off the southern California coast.
The gunfire testing came at about the three-quarters point of the System Design and
Development (SDD) test program. The test aircraft (2AF:0002 or “AF-2”) was the highest time
JSF jet with considerable envelope expansion in its log. During all previous AF-2 testing a gun
had been installed with no ill-effects from the inertia loading of the supporting structure. No
attempt was made to refine the indicated aiming point for gunfire, especially as the test aircraft
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47th Annual International Symposium 08 – 11 August, 2016, Wichita, KS AF-2 was not equipped with a fully representative system. Following these tests, a Mission
Systems (MS) gunnery campaign is planned to assess effectiveness of aiming and accuracy of
the system.
Engineering development test objectives were:
 Demonstrate functionality and safe operation of the system throughout the operating
envelope
 Collect impact dispersion data on the ground
 Validate minimum ventilation airflow predictions
 Verify suitable load-bearing capability of related structure
 Collect structural data to evaluate thermal, vibration, acoustic, and coating erosion
effects
 Collect engine health data to assess the effects of gunfire
 Test pilot to qualitatively assess gunfire effects on aircraft and systems operation
 Ride-along collection of loads, vibration, and acoustic data on various stores
Hazards mitigated by planning and test procedures were inadvertent/unintentional firing, gun
jam/misfire/hang fire/ammunition explosive event, test and/or chase aircraft struck by a round,
engine stall/flameout, and structure limit exceedance. These are in line with historical hazards
from gunfire testing, with some unfortunate events for illustration. As part of the mitigations,
Safety of Test (SOT) parameters were monitored real-time. Given the high firing rate (nominally
3000 rounds per minute) plus reaction time, it was impractical to give terminate calls once firing
commenced. Consequently, the SOT measurands were verified prior to firing and results
checked post-firing for point-to-point clearance. The effects of engine ingestion of gun gas or
blast pressure wave were monitored via existing engine flight test instrumentation and bus
parameters. The engine was designed to self-recover from such events, but emergency
procedures existed to deal with stall/surge or flameout eventualities.
No significant flight dynamics transient was expected from airborne gunfire. The nose boom was
removed for gunfire. Hazard patterns for airborne gunfire were prepared by China Lake and Sea
Test Range experts.
4.0 SYSTEM DESCRIPTION
The Gun Airborne Unit GAU-22/A four-barrel 25mm Gatling gun has a 181-round Linear
Linkless Ammunition Handling System (LLAHS) supporting the high firing rate (3.6 sec to an
empty magazine). Operationally, this was to provide three firing passes against targets. All tests
employed Projective Gun Unit PGU-23 Target Practice (TP) rounds or PGU-24 dummy rounds.
The gun is hydraulically driven and the ammunition percussion primed. A burst length limiter is
set to fire a specific number of rounds, ±5, as verified via a rounds counter.
Firing builds up various gases in the gun bay. The bay is designed to provide a flow of
evacuation ram air in through the purge door scoop and the adjoining plenum into the front of
the gun bay and into the breech/ammo area, all then out a flush breech exit vent screen at the
aft end of the bay (Figures 1 and 2). The primary element in the gases is hydrogen which was
required to be kept to less than 18 percent concentration via ventilation to prevent an explosive
event in the presence of an ignition source. Partial burning of the gas at lower concentrations
during firing is common, with a visible exhaust flame. With gun doors closed, there is a trickle of
air into a flush screen aft of the purge door and out the aft screen.
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47th Annual International Symposium 08 – 11 August, 2016, Wichita, KS Figure 1. General Gun Component Arrangement for F-35A
Figure 2. Gun System Elements
The gun is highly integrated into the systems of the F-35A. The Master Arm switch has to be
selected ON and the gun selected on the Stores Management System (SMS) Multi-Function
Display (MFD). Either air-to-air (“Dogfight”) or air-to-surface mode must be selected via either
MFD or Hands-on Stick-and-Throttle (HOTAS) switch. The Gun System Control Unit (GSCU)
communicates with the SMS, where ammunition must be shown loaded, and reports gun/door
status to other onboard computers. At the trigger second detent the doors snap open and firing
begins, ramping up to full rate in 0.4sec. Once the pilot releases the trigger the unfired rounds
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47th Annual International Symposium 08 – 11 August, 2016, Wichita, KS are counted while passing through the gun followed by a reverse clearing action that moves the
ammunition to the handling system. All this occurs in 0.75sec during which any retrigger
command is delayed before firing resumes. The doors close in about 5sec to provide adequate
ventilation. Adverse system status is posted via the Integrated Caution, Advisory, and Warning
Systems (ICAWS).
During development tests the gun fired some 150,000 rounds without a jam or misfire – the
latter being among the highest design load condition for the gun structural installation. The gun
system was qualified in 2008 via ground tests, with the gun on a stand, during which more than
36,000 rounds were fired (approximately two life times). The opportunity was taken to collected
loads, vibration, and temperature data to inform the aircraft detailed design effort.
The gun is supported within the aircraft by two recoil spindles at the breech end and a muzzle
sleeve at the forward end of the barrel assembly (Figure 3). The assembly rotates within the
sleeve that also reacts off-axis radial firing loads (round out of barrel offset from the assembly
rotational axis). For installed testing, gunfire loads were to be measured via calibrated strain
gauges on each spindle and on the muzzle sleeve.
MUZZLE SLEEVE
RECOIL SPINDLES
(aft mounts)
Figure 3. Loads Instrumented Gun Support Elements
A flight test-unique Gun Door Switch (Figure 4) permitted operating the gun doors separate from
the gun system. For ground testing, four gas sample bottles were evacuated to near-vacuum
and individual valves opened for 0.5 sec to suck in gasses via a sequencer triggered by gun
signals. One bottle opened about 0.9sec after trigger pull and the others 0.75sec after the end
of the burst. The sampling tubes from the battles ran to optimal points in the plenum and gun
bay as suggested by CFD. Airflow instrumentation within the gun plenum, gun bay, and ammo
bay consisted of total and static pressures and temperatures. Temperature-sensitive paint and
adhesive dot recording tapes (‘temp tabs’) were also added at several points on and around the
gun. Dynamics data transducers (accelerometers, microphones, thermocouples) were also
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47th Annual International Symposium 08 – 11 August, 2016, Wichita, KS installed on and around the gun to collect data to characterize the environment and assist
problem resolution.
Figure 4. Gun Port / Purge Door Cockpit Switch
5.0 TEST PREPARATION
Based upon experience with the F/A-22 (Reference 1) and the new test approach being
employed, a risk reduction measure was conducting an airborne airflow check for suitable
ventilation and for CFD validation. Data were collected at 35 flight conditions with the doors
opened via the flight test switch. Airspeeds were from the minimum sustainable airspeed to the
maximum Mach and safe dynamic pressure (q, at 6000ft altitude) and a selection of operational
altitudes. At each condition, 1-g trim and sideslip sweeps were performed. Several also had
push-over and pull-ups to maximum and minimum Nz. The airflow survey immediately
uncovered several problems with the instrumentation and so changes and recalibration were
followed by regression testing such that the entire survey was flown approximately two times
over a three-month period.
Loads data were also collected on the muzzle and purge door actuators or links during the
ventilation testing. The purge door loads initially did not make sense but this was corrected by
using a different strain gauge bridge arrangement. The peak loads on the doors occur on
closing. There were large margins for gunfire, though the purge door (not expected to be
affected by gun blast) was near limits for closing at the highest q flown (around 1550 psf versus
the absolute maximum of approximately 1650 psf). However, structural analysts determined
there was sufficient structural capability to extend the limit load on the purge door. Additional
loads from sideslips were almost imperceptible. An accelerometer on the muzzle door also
provided dynamics data.
As the initial A-model test aircraft were being assembled it was discovered via the gun
qualification testing that gunfire loads and vibration levels were greater than initially estimated.
Consequently, the design incorporated into the SDD jets was inadequate. While production F35As incorporated a revised design, firing the gun from AF-2 required an extensive modification
supported by design and a parts/tool kit. A strengthened muzzle door was also incorporated.
This took the aircraft out of test for approximately four months after the ventilation flights.
The modification also permitted air leak paths to be ‘plugged’ to ensure the best possible gun
gas evacuation. A ground airflow test was performed to ensure the purge fan planned to be
used for ground test would pull the minimum required flow (replicating that at the pinch point)
and to check airflow instrumentation functionality. These results were compared with one before
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47th Annual International Symposium 08 – 11 August, 2016, Wichita, KS the mod and it was observed that less airflow was sustained. This suggested further problems
with the instrumentation and so additional airborne ventilation testing was added to obtain “truth”
data before beginning the airborne gunfire testing. A single flight found acceptable airflow.
Qualification of aircraft components to the expected gunfire vibration spectrum had cleared all
but the Vehicle Management Computers (VMC) and the 270-Volt battery, both suffering failures
in the ‘shake-‘n-bake’ lab tests. VMCs were limited to a small number of units that had been
improperly manufactured by the vendor and those planned for the gunfire testing were cleared.
Revised batteries were made available but with a limited life in the gunfire environment,
mandating periodic removal during airborne gunfire. VMC and battery vibration levels were
recorded via accelerometers during ground testing for comparison with the lab test levels.
P&W took advantage of the ground test to collect vibration data in the present of gunfire with
added non-flight instrumentation to compare vibration levels with the spec. Instrumentation was
also added to the muzzle door to collect vibroacoustics data, the wires running across the
aircraft to stand-alone recording and analysis equipment. Some of the instrumentation was
found to have a short life under the severe gun blast environment. On the first shot bits were
seen flying down range and it took several shots before a survivable installation was
established.
6.0 GROUND TESTING
For ground gunfire the aircraft was secured 996 ft from the gun butt (Figure 5). A 10x10 ft
canvas target over a wooden frame was slung across the face of the butt. Although perfecting
the aiming of the system was not a goal, dispersion data was collected by counting holes and
distance from a corner of the target to determine a mean impact point. To prevent aircraft
motion on gear, the defueled aircraft was placed on jacks and then moored by ten chains to tiedown points specially buried in the pavement. The engine and Integrated Power Package were
not operated and all systems were sustained by ground support equipment. A ground test
station was used to ‘fool’ the aircraft systems into assessing itself as airborne to permit ground
gunfire. The gun does not have a boresighting feature, so the aircraft was aligned with butt and
the target adjusted to match a boresight. Because of the height of the aircraft on jacks, the usual
bulk loader could not be used and all rounds were individually hand-loaded.
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47th Annual International Symposium 08 – 11 August, 2016, Wichita, KS Figure 5. Aircraft Secured at Gun Butt and Showing Adapter Top Liberation
As common for such events, a stay-out zone ahead and in a 60 deg cone to the sides of the
loaded gun was policed by the Range Safety Officer (RSO) who also coordinated with the pilot,
control tower (to prevent aircraft crossing through cone out to ballistic range of the projectiles),
ground personnel, and the control room for permission to fire. The pilot had to mount and
dismount the jet from the starboard side, opposite the loaded gun.
Planning converged on 12 test points with nine individual firing events. This built-up in number
of rounds fired from 10 to the full ‘can’ of 181 rounds, initially with no sampling bottles and the
gun bay exposed by panel removal. In the panel-removed configuration, ground cooling cart air
was blown across the exposed gun to clear away gun gases. The engine nacelle fan was also
turned on to evacuate any gases that may have accumulated. Firing then continued with panels
and sample bottles installed. It ended with rapid retrigger events. A mobile control room was
positioned beside the jet and data passed via an instrumentation cable instead of telemetry
(TM). The side of the canopy adjacent to the gunfire path was partially covered with a “witness
cover” to collect traces of firing particles and residue that might degrade the canopy over time,
and also to protect the transparency. No marks attributed to the gun firing were observed from
the witness cover which would convulse violently from the blasts during testing.
After days getting the entire test setup working suitably, the trials began by checking the many
safety interlock features of the system via a matrix of 19 conditions. The first shot, capping
approximately nine months of planning and preparation, provided highly sought-after and
valuable data. After a long, hot day, the shot occurred with just 10 minutes remaining before
range time expiration. A short time after the shot the control room generator quit as it burned the
last of its fuel. The entire team felt jubilant and fascinated with the new data.
The gun bay panels and vacuum sample bottles were then installed, ventilation air was provided
via an extraction purge fan drawing air from an adapter box fitted over the exit screen (Figure 6).
The purge fan adapter was attached with the panel fasteners and had a lid designed to pop off
at 2 psi should excessive combustion of gases occur within the gun bay. The fan was run for 15
minutes following a shot to assist in venting and also cooling the gun and structure for post-firing
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47th Annual International Symposium 08 – 11 August, 2016, Wichita, KS inspection. The flow, as measured via the instrumentation, had to meet a minimum based on
CFD. The gas bottles were passed to an on-base lab for determination of the concentration of
the constituent chemical elements. The analysis had to be performed within 48 hours or it was
expected the gases would leak away. The lab had a bit of learning curve for personnel and
equipment, and so initial results were slow in coming and problematic. This improved rapidly
and eventually consecutive day firings were executed. The purge door and the adjacent Ground
Power/ Maintenance Door would interfere or ‘clash’ if the latter was opened when the gun doors
were opened. (It happened twice, though not during the gun testing, damaging both doors
beyond repair.) So, for the ground tests it was best to simply remove the maintenance door.
Figure 6. Gas Bottle and Fan Adapter Ground Test Installation
As usual for any new test methodology, numerous issues cropped up and had to be overcome,
and so testing initially proceeded slowly and with some regression. The testing was expected to
take two weeks but extended to three months and 12 firing events with 865 rounds expended.
The aircraft was removed and then reinstalled at the gun butt several times. After the third test
firing and boresight readjustment, all rounds hit the target board. Vibration levels on the VMCs
and battery were within the specification limits.
On the first attempt to perform back-to-back shots, the system failed to fire minutes after the first
15 rounds went out. A nuisance power fault, caused by a software bug, caused a brief gun fault
that self-cleared but still prevented firing after the Master Arm was selected OFF. The
workaround was to select OFF within 5 sec of initial trigger release (the period before the purge
door closed), or to cycle a Master Integrated Core Processor (ICP). The former appeared the
best option with less potential for surprises. The test was repeated successfully. This
workaround was considered acceptable for flight as well, but not desirable during loaded
maneuvers where the pilot would need to integrate that task along with many others such as
avoiding ground impact. Practicing the technique during the remaining ground gunfire found that
approximately half the time the pilot was so startled by the noise and vibration of the firing that
he failed to complete the task in the allotted time. The ICP cycle appeared a better option in
flight for dynamic maneuvers. Additionally, the flight test Gun Door Switch command contested
with the GSCU commands, due to a non-optimal implementation, and so the door repeatedly
opened and closed on a 16 sec period when employing the switch. On the ground this was
avoided by entering zero rounds in to the SMS then putting the maintenance interface panel
state to TEST to put the GSCU ‘to sleep.’ This would not be possible in flight where the rounds
would be entered before leaving the chocks.
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47th Annual International Symposium 08 – 11 August, 2016, Wichita, KS On the first 90-round shot the top popped off the purge fan adapter above the gun bay exit
screen approximately 0.3sec after the final round was fired (Figure 5). Although the 10 lbs cap
had two restraining steel straps, these failed and the lid flew about 40 ft from the aircraft.
Although analysis was not conclusive, best determination was that gases had accumulated in a
low-flow zone within the adapter box (confirmed by CFD) and these were ignited by an unknown
source. The box was redesigned to provide a more direct path around the 90 deg turn from the
screen to the fan duct and the top was retained by netting attached under the panel fasteners. A
larger diameter duct and hose to the fan was also adopted. The pressure to pop the top
increased to 3 psi but still below the 5 psi proof pressure of the gun bay. Changes to
instrumentation were introduced to provide additional insight into such an event should it
reoccur. The ‘top liberation event’ (an unidentified hazard) drove a greater distance from the
aircraft for observers to 200 ft. Because the event occurred before all bottles had sampled, that
test point had to be repeated and no similar problem occurred for the remainder of the testing.
When the team performed the rapid retrigger test, and fired out the full magazine, they saw that
the barrels continued to spin after the last round was fired and until the pilot released the trigger.
This was because the system had not completed the reverse-clear process after the 41 rounds
had fired and when the test pilot pressed the trigger the second time. Consequently, the round
counter and SMS did not continue counting down during the retrigger and continued to try firing
the perceived 140 rounds remaining. This deficiency was noted but did not prevent continuing
with testing. It was also learned that emptying the magazine led to the possibility of the GSCU
reporting more rounds then actually fired or remaining as it attempts to fire any miscounted
rounds (count only had to be ±5). The aircraft software, that accounts for mass properties
change of rounds expended (0.69 lbs each or a total 125 lbs), had difficulty if this was greater
than actually fired. This would generate a fault and so had to be addressed.
Both the muzzle sleeve and recoil spindle measurements provided an independent means of
counting rounds fired before the ‘brass’ was unloaded and counted. Loads measurement of
door actuator/link responses were another means of checking door position since there was no
separate indication. The sleeve and spindle loads exceeded limits on the first shot, but
conservatism was revisited and new limits provided. The muzzle sleeve limits (simply strain)
were then exceeded due to thermal strain increasing as more rounds were fired. It became clear
that these would remain problematic but the large strength margin on failure loads for this nonflight critical structure meant that real-time monitoring was not necessary. Other than ensuring
that the strain measurements were functioning, derivation of the sleeve loads was left to posttest analysis which did not need to be performed shot-to-shot. The next surprise was failure of
sleeve strain gauge responses due to the high heating of the steel sleeve during long bursts.
The heat was high enough to burn away the epoxy paint and primer at the top and bottom of the
front end, melt solder joints, and disbond a gauge. The sleeve had to be removed and an
attempt made to relocated the temperature-compensating gauges to the aft end of the
component. This was not successful. Some data was lost from the final tests but the loads were
consistent shot-to-shot. No other ill-effects of the high temperatures were experienced.
The discoveries during the testing of how the system “really works” and the required
workarounds to confidently permit multiple shots on a sortie raised the question of
representativeness of the subsequent flight gunfire tests to follow. It was greatly desired to
introduce software changes to correct the deficiencies. Given that it took months of intensive
work to produce any new software build to load onto the jet, regardless of perceived complexity,
this raised program cost, schedule, and resource issues. It would mean interrupting the airborne
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47th Annual International Symposium 08 – 11 August, 2016, Wichita, KS gunfire after two flights of 1-g shots or perform future regression testing. Ultimately, the decision
was made not to wait for improved software. Regression testing, probably via reconstituting the
ground gunfire setup, was also rejected in favor of systems integration lab testing of the
software.
7.0 AIRBORNE TESTING
Planning converged on 21 test points or individual firing events over eight flights and expected
to take two weeks, with most involving multiple gunfire events. The flights were planned with
various internal and external store configurations, many of these inert stores were instrumented
to collect loads, vibration, and acoustics data as a ride-along objective. Obtaining and qualifying
each of these stores for flight on the aircraft in approved configurations was an effort to get an
early look at environmental weapon data but was not permitted to impede progress to
completing the primary gunfire objectives and returning to other priority SDD testing.
The planned flight conditions (Figure 8) are much like those for the earlier ventilation testing.
They built-up to least ventilation ram pressure and potentially greater concentration of explosive
gases via number of rounds fired (from 30 to the full ‘can’ of 181 rounds) and flight conditions
from middle of the Mach-altitude envelope to the pinch point (upper left). The buildup in
structural loads was via increasing dynamic pressure (lower right). Most tests were at 1-g trim,
although several required dives to attain the necessary energy state to sustain the condition for
the duration of the firing. Loaded maneuvers, both positive and negative Nz, were planned to
demonstrate firing under such conditions per the specification. The number of firing events per
flight was limited by the 181 rounds available and the desire to fire at least 120 rounds for each
unique store loaded.
A ‘cold pass’ for practice of radio calls, chase positioning (safety and photography), and
airspace adherence also permitted a check for suitable airflow by opening the gun doors. The
values had to meet minimum requirements before the ‘hot’ pass. The door cycling seen on the
ground was also present in flight while using the Gun Door Switch with the GSCU powered
(rounds loaded) during the cold pass. While this was not expected to prevent suitable collection
of airflow data at 1g, there was concern about the maneuvering test points. However,
performing a separate 1-g cold pass at those flight conditions was suitable.
As expected, the first inflight gunfire test (Figure 8) was insightful but gratefully devoid of
surprises. The test pilot noticed a fraction of a second delay from trigger pull to firing (gun spinup). Vibration levels were similar to the F-16, although with a deeper “rumble”, and no muzzle
flash was visible from within the cockpit (though visible to chase). The control room only
recorded approximately 25 percent roll stick input and roughly 2 kts decrease in airspeed (also
observable by chase). The engine suffered no detectable ill effects of the gunfire.
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47th Annual International Symposium 08 – 11 August, 2016, Wichita, KS Figure 7. Airborne Testing Matrix and Flow
In the second airborne gunfire test flight, the aircraft returned without executing any test points
due to an unanticipated weapons display and selectable air-to-air missiles associated with ridealong stores when setting up to fire the gun. For the next flight, anti-jettison devices were added
to the missiles and all planned gunfire events were completed, included gunfire at the high, slow
“pinch point.” No ventilation, engine issues, or other anomalies were noted.
After testing was underway an analyst, looking at initial results and plans for the next flight,
wondered why we were jumping from subsonic to supersonic conditions. No transonic points
had been planned because none had been thought necessary. The analyst pointed to test
results from a legacy program that suggested higher muzzle door loads from transonic shots
because of different blast pressure wave propagation around the muzzle door than at subsonic
or supersonic conditions. This information was in a report not accessible to the F-35A test
planners. Subsequently, two transonic shots points were added, a buildup and one near the
maximum q-bar, and the sequence of test points adjusted.
During the third flight, similar to an event on the ground, a software ‘feature’ prevent a retrigger
event. In this case our ICP cycle workaround reset the initial round count in the GSCU but not
the cumulative rounds fired and so the latter was greater than the initial load. Consequently, the
GSCU set the rounds remaining count to zero and the gun did not fire. The correction was
introduced in a later software build. Additionally, the need for the Master Arm or ICP cycle
workaround was negated by a software update.
At the time of writing this paper, only five of the eight planned flights have been executed (likely
to be nine with one flight truncated to a single 60-round shot by range time). Multiple delays
associated with aircraft modifications, higher priority testing, and software integration anomalies
have delayed testing by many months.
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47th Annual International Symposium 08 – 11 August, 2016, Wichita, KS 8.0 REGRESSION TESTS
Given the need for software changes uncovered during the gunfire testing, some regression
testing was assessed as necessary before delving into the mission systems testing focusing on
aiming and accuracy. This testing was performed on the jet dedicated to the MS work and so
devoid of the instrumentation unique to AF-2.
9.0 CONCLUSIONS AND RECOMMENDATIONS
The gun installation and integration for the F-35A was shown as suitable and effective within the
limits of the development test program in the flights conducted up to this point. The burst length
limiter was good to ±1 round. Round impact dispersion met the specification value. Some nonrepresentative features were known at the outset, including the test aircraft not equipped for
gunfire aiming, but deficiencies in software integration were uncovered during the campaign.
While these did not impede completing the tests, they put in question the representative nature
of the configuration. This will necessitate regression testing prior to mission systems targeting
tests.
Figure 8. Airborne Gunfire Event
The method of collecting gun gas samples during ground gunfire to validate CFD predictions of
suitable ventilation airflow, with a minimum identified and verified in flight before firing, proved
successful. It cleared the aircraft for airborne gunfire with no adverse effects associated with
gun gas concentration. Hence, a new methodology for gun integration and testing for a low
observable aircraft has been demonstrated. Modeling and simulation and a novel test setup
were used in new and different ways to help save money and increase test efficiency with
airborne gunfire testing. This is a very important lesson in today’s fiscal environment. Many
programs can apply this method to increase test efficiency and reduce overall costs while
maintaining a high degree of accuracy. The adjustment to the ground setup following the purge
fan adapter lid liberation event was also successful and taught a lesson about design of such
test equipment.
The late addition of the transonic gunfire points is an example of all sources of lessons learned
from a similar past effort being unavailable, even in a redacted form, to the testers planning the
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47th Annual International Symposium 08 – 11 August, 2016, Wichita, KS project. All participants should point to any source information and perhaps assist in obtaining
same. “You don’t know what you don’t know”, but every effort should be made to lift the veil. It is
in the interest of organizations providing data to the same customer (in this case the USAF)
holding compartmentalized information to assist in dissemination in some form to assist test
efficiency and effectiveness of projects that follow.
The push to integrate collection of vibroacoustics data from instrumented stores in the presence
of gunfire with the baseline gunfire testing was a commendable effort at test efficiency.
However, as the first airborne shot approach it became clear this added an inordinate amount of
risk, (some stores were non-jettisonable and some had never been flown on the test aircraft).
Ultimately it was decided to perform the first gunfire flight without stores to minimize risks
associated with basic unknowns before adding to test complexity. The second flight, with such
an instrumented store, returned without shooting because of a store malfunction. It is important
that the added complexity and so risk of concurrent or ride-along testing be assessed when
undertaking a fundamentally new activity with the test aircraft.
The integration of the gun system with so much of the other avionics aboard the airplane made
for an exceptionally complex means for delivering rounds on target. At every step of the testing
the team was learning how the machine “really worked” and adjusted the testing as necessary.
Integrating systems into other highly complex systems, especially those heavily reliant on
software, will lead to unexpected behaviors and interactions. These complex interactions and
unexpected outcomes will lead to delays in the planned timeline. Although test completion took
longer than originally expected, ultimately, this is how development testing is supposed to work;
delivering data and information to the designers for system optimization and improvement.
Rationally assessing schedule and technical risk of a test effort, in addition to safety risk, must
drive test and buildup choices. This was done well for the F-35A gunfire project, but testing
surprises arose repeatedly to greatly extend the effort. As usual, documentation, review, and
approval of the testing and changes consumed more time than actual preparation and
execution. Nonetheless, a good team effort overcame all challenges up to this point.
10.0 REFERENCES
Rebello, Vanessa, Major, USAF, Huffman, Richard, Captain, USAF, Brown, James, III,
Lockheed Martin, The Raptor is Now a Fighter: F/A-22 Gun Integration Testing, Society of
Experimental Test Pilots, Proceedings of the 47th Annual SETP Symposium, 2003.
11.0 BIOGRAPHY
William J. Norton is a flight test engineer working in southern California and at the Air
Force Test Center. This continues a career began during 20 years as a US Air Force
officer, when he also served as aircrew on test and support aircraft. Bill has held
numerous positions in many organizations on more than two dozen aerospace
programs spanning all aircraft types, including fighters, bombers, VSTOL, missiles, and
research. He has penned scores of technical papers, a multitude of magazine articles,
and a dozen books. Bill is a civil pilot with numerous ratings, has restored and operates
a DHC-1 Chipmunk, and built a Rutan Long-EZ. He holds a Masters in Aeronautical
Engineering.
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