The document evaluates the Aero L-39C aircraft to determine its suitability for the South African Air Force as an advanced jet trainer. Various human factors, systems, performance metrics, and stability characteristics were assessed, ultimately concluding that the aircraft was unsuitable for the specified mission due to unacceptable stall characteristics. Recommendations were provided to improve certain design flaws and issues identified during testing.

1. 1
Student Flight Test Engineer: Instructors:
Barend Bezuidenhout Russ Stewart / Ed Solski
LIMITED EVALUATION
AERO L39C

2. 2
Purpose
• To conduct a limited evaluation on the L39C to
determine its suitability for the Advanced Jet
Trainer mission in the South African Air Force.

3. 3
Scope
• Test Objective 1: Human Factors
• Test Objective 2: Systems
• Test Objective 3: Take off and Landing
• Test Objective 4: Stall
• Test Objective 5: Performance
• Test Objective 6: Static Stability
• Test Objective 7: Dynamic Stability
• Test Objective 8: CLHQ

4. 4
Test Item Description
• Aero L39C (N57XJ)
4
Aspect Technical Details
Engines
Al-25 TL twin shaft turbofan with bypass
ratio of 2.0 and static thrust of 3,700 lbs
Wing
Area
202 ft² (18.8 m²)
Wingspan 31 ft (9.46 m)
Length 39 ft 10 in (12.13 m)
Height 15 ft 5 in (4.77 m)
Zero Fuel
Weight
8,020 lb (3,637 kg)
Ramp
weight
10,298 lb (4,671 kg)
Control
System
Reversible controls

5. 5
Instrumentation
• Cockpit Instruments
• Handwritten data
• Digital Stopwatch
• Voice and Video Recorder
• Handheld Force Gauge

6. 6
Test Team
• Instructors: Russ Stewart
Ed Solski
• Pilot: David Macaluso (Teton Aviation)
• TC / FTE: Barend Bezuidenhout

7. 7
Test Limitations
• 4 sorties (or 6 hours flying time) were available for
airborne evaluation.
• Maximum braking and intentional high speed aborts
were not permitted.
• Test had to be done to conservative buffers to all
flight manual limitations
• Spins or Phase C/D stalls were not permitted.

8. 8
Test Assumptions
• Standard Weight: 10,298 lbs
• Mid Mission Weight: 8,900 lbs
• Main emphasis of evaluation on Performance and Flying Qualities
Test Location and Conditions
• Mojave, CA
• Isabella MOA
• 2505
• Sortie 1: Calm and clear
• Sortie 2: Windy and clear
• Sortie 3: Windy and partly cloudy
• Sortie 4: Windy and partly cloudy

9. 9
Mission: Advanced Jet Trainer

10. 10
Mission: Profile
Landing
Low Level
A/G
Cannon
Vy
Climb
Endurance
Cruise
ACM
Recce
Range
Cruise
PFL
Short Field
Stop-and-go
Take off
CAP Sweep
Instrument
ApproachT1
T2
60
nm
40
nm
20,000 ft
10,000 ft

11. 11
Result:
Field of View diagram
Test Objective 1 – Human Factors

12. 12
Test Objective 1
Human Factors
Small and
obscured
labeling
Obscured
Fuel Meter
Obscured
EGT
Obscured
RPM

13. 13
Test Objective 1 – Human Factors
Conclusion:
• The Front Cockpit Layout of the aircraft was Marginally Acceptable.
Mission:
• Low level Air to Ground attack
• Head outside mostly
• Don’t notice warning until climbing away
• Fuel, fire, engine overtemp etc.

14. 14
Test Objective 1 – Human Factors
Recommendations:
• The absence of a master warning light should be rectified. [R-2]
• The low brightness level of the Caution and Warning panel lights
should be rectified. [R-3]
• The bezel obscuration of the Fuel Remaining, EGT and %RPM
gauges should be rectified. [R-4]
• The small labeling on the Fire Detection Test and EGT Regulator Test
panel should be rectified. [R-6]

15. 15
Test Item Description
4

16. 16
Aviate – provides necessary speed, altitude and attitude information
Navigate – provides current position, guidance to waypoint, flight plan
following and instrument approach
Communicate – intercom, radio (monitor two freqs while talking on
one)
Test Objective 2 – Systems

17. 17
Test Objective 3 – Takeoff and
Landing
Test Method:
• Take Off Take Off Performance
• Landing Performance
• Data corrected for:
~ for wind,
~ density,
~ runway slope
~ weight (9,920 lbs)

18. 18
Result:
Take off bar chart
Test Objective 3 – Takeoff and
Landing

19. 19
Result:
Landing bar chart
Test Objective 3 – Takeoff and
Landing

20. 20
Test Objective 4 – Stall
Test Method:
• Practical sequence
~ Clean (Idle 1, 3 kts/sec ; PLF 1, 3 kts/sec)
~ Landing Config (Idle 1, 3 kts/sec ; PLF 1, 3 kts/sec)
• Accelerated stall
• Clean (200 kts, PLF 3 kts/sec, 2-3.5 g, Left, Right)
• Clean (200 kts, PLF 5-9 kts/sec, 3.5 g, Left, Right)
• Phase B stall
~ Clean (Idle 1kts/sec, Phase B inputs for 1 sec, Left, Right)

21. 21
Test Objective 4 – Stall
Power
Bleed
Rate
Stall
Warning
(kts)
Stall
Speed
(kts)
Δ Δ
Idle 1 kts/sec 112 109 -15° -2°
Idle 3 kts/sec 110 105 -30° -4°
PLF 1 kts/sec 110 98 -45° -4°
PLF 3 kts/sec 105 95 -20° -3°
Power
Bleed
Rate
Stall
Warning
(kts)
Stall
Speed
(kts)
Δ Δ
Idle 1 kts/sec 98 93 -5° 0°
Idle 3 kts/sec 96 91 -15° -2°
PLF 1 kts/sec 95 90 -10° -1°
PLF 3 kts/sec 93 89 -15° 0°
Clean
Config
Landing
Config

22. 22
Test Objective 4 – Stall
Accelerated Stall
Bleed
Rate
g Dir Δ Δ
Comments
3
kts/sec
2
Left 0° -5 Benign
Right -10° -2 Benign
3
Left 45° 5 Manageable
Right 30° 10 Manageable
5-8
kts/sec
3.5 Left 175° 20 Unpredictable
3.5 Right 360° / -90° 15 / -10
Totally unpredictable

23. 23
Test Objective 4 – Stall
Conclusion:
• The Accelerated Stall characteristics were Unacceptable.
Mission:
• Break off from A/G attack
• Break over airfield to join circuit
Recommendations:
• The unpredictable rolling of the aircraft under high g and high speed
bleed rates must be rectified [R-1].

24. 24
Test Objective 5 – Performance
Test Method:
• Climb (10,000 ft)
~ Level Acceleration, 10,000 ft
~ Saw tooth climb, 9,000 – 11,000 ft
• Cruise (20,000 ft)
~ Vh, slow down in 20 kts increments to 140 kts
~ take fuel flow and RPM each time
• Turn (15,000 ft)
~ Vh, set g (1.5 +.5 + … up to 4 g)
~ Speed Stabilizes
• Engine Out Glide (10,000 ft)

25. 25
Result:
Climb results
Test Objective 5 – Performance

26. 26
Result:
Cruise results
Test Objective 5 – Performance

27. 27
Result:
Cruise results
Test Objective 5 – Performance

28. 28
Result:
Cruise results (Map with range)
Endurance value
Test Objective 5 – Performance
Range Capability = 23 Nm |climb + 459 Nm |cruise = 482 Nm

29. 29
Result:
Turn results
Test Objective 5 – Performance

30. 30
Engine Out Glide Performance

31. 31
Test Objective 6 – Static Stability
Test Method:
• Longitudinal Static Stability
• Lateral Directional Static Stability
• Maneuver stability

32. 32
Result:
• Longitudinal Static Stability
Test Objective 6 – Static Stability

33. 33
Result:
• Lateral Directional Static Stability
Test Objective 6 – Static Stability

34. 34
Result:
• Lateral Directional Static Stability
Test Objective 6 – Static Stability

35. 35
Result:
• Maneuver stability
Test Objective 6 – Static Stability

36. 36
Test Objective 7 – Dynamic Stability
Test Method:
• Short Period
• Phugoid
• Dutch Roll
• Spiral Mode
• Roll Oscillations

37. 37
Result:
• Short Period (thumbprint graph)
Test Objective 7 – Dynamic Stability
L39C (N57XJ)
28 Nov 12
(09h30)
Mojave, CA
15,000 ft (PA)
OAT -3°C
8,900 lbs
26.7 % MAC
Clean Config
(250 kts)
Landing
Config
(130 kts)

38. 38
L39C (N57XJ)
28 Nov 12 (09h30)
Mojave, CA
15,000 ft (PA)
OAT -3°C
8,900 lbs
26.7% MAC
212
292
222
288 282
219
40 2:00
2:38
3:19
4:00
290
1:19
Clean
Config
Phugoid
d 0.079
rad/sec
 0.02
n 0.079
rad/sec
d 0.128
rad/sec
 0.10
n 0.129
rad/sec
115
150
125
147 139
118
25 1:34
1:56
2:23
2:39
155
1:10
Landing
Config

39. 39
Result:
• Dutch Roll (recreated damping, phi/beta video)
Test Objective 7 – Dynamic Stability
Clean Config
d 4.34 rad/sec
 0.1
n 4.36 rad/sec
Landing Config
d 2.14 rad/sec
 0.3
n 2.24 rad/sec

40. 40
Result:
• Spiral Mode (graphs)
Test Objective 7 – Dynamic Stability

41. 41
Conclusion:
• The Spiral Mode Characteristics were Marginally Acceptable.
Mission:
• Times when pilot is heads-down or in IFR during cruise phase of
mission.
Recommendations:
• The bank angle roll-off to the right in the Spiral Mode should be
corrected. [R-5]
Test Objective 7 – Dynamic Stability

42. 42
Test Objective 8 – CLHQ
Test Method:
• Taxi
• A/G
• Pitch capture
• Instrument approach

43. 43
Results:
Test Objective 8 – CLHQ
Task Criteria Cooper
Harper
Rating
Capture centerline within 10 sec at
5 and 15 kts
Desired: 1 overshoot
Adequate: 2 overshoots
2
Aim at an Air to Ground Target for 3
seconds.
Desired: 5 mils
Adequate: 10 mils
3
Capture pitch attitudes in 10°
increments, each time within 2
seconds.
Desired: 1 overshoot
Adequate: 2 overshoots
3
Fly an instrument approach and
follow the horizontal guidance.
Desired: 1 dot horizontally
Adequate: 2 dots
horizontally
2

44. 44
Recommendations
[R-1] The unpredictable rolling of the aircraft under high g and high
speed bleed rates must be rectified.
[R-2] The absence of a master warning light should be rectified.
[R-3] The low brightness level of the Caution and Warning panel
lights should be rectified.
[R-4] The bezel obscuration of the Fuel Remaining, EGT and %RPM
gauges should be rectified.
[R-5] The bank angle roll-off to the right in the Spiral Mode should
be corrected.
[R-6] The small labeling on the Fire Detection Test and EGT
Regulator Test panel should be rectified.

45. 45
Overall Conclusion
The L39C was found to be NOT SUITABLE for the mission specified
in the ORD.
It is recommended that the aircraft is NOT selected as the Advanced
Jet Trainer for the South African Air Force, unless Accelerated Stall
Characteristics [R-1] can be rectified.

46. 46
Lessons Learned
• Use document formats that is in context with the
task that was given.
• Extra effort with test cards normally produce a
better success rate in the air.
• Pilot + Coffee + Tact  Good Data

47. 47
Questions / Remarks

48. 48
More Test Item Description Details

49. 49
Range Capability = 23 Nm |climb + 459 Nm |cruise = 482 Nm
20,000 ft
210 kts
(1,351 lbs fuel)
459 Nm
300 lbs
available for
landing
Fuel reserve for 30 min
@ best endurance
speed
(360 lbs fuel)
23 Nm
2,278 lbs
Langebaanweg
200 kts climb
(267 lbs fuel)
Cruise Range Profile

50. 50