A-10A Test Jet, 73-1666

[Old Viper Tester]

One of my A-10 flight tests in 1984-85 with the 6512th Test Squadron. The A-10 Combined Test Force at Edwards AFB was shut down at the end of 1983. The remaining A-10 test programs and two A-10s were turned over to the 6512th. When the test programs were completed in mid-1985, the A-10s were sent to the 3246th Test Wing at Eglin AFB as part of a consolidation of Systems Command A-7Ds at Edwards and A-10s at Eglin.

 

'666 was my Ground Proximity Warning System (GPWS, pronounced jip-wiz) test jet. A program aimed at reducing controlled flight into terrain incidents. The A-10 GPWS consisted of a black box processing radar altimeter (RADALT) data and making voice and HUD warnings. Because of the low altitude maneuvering of the A-10A in prosecuting close air support, RADALT transmit and receive antennas were installed on both vertical tails as well as under the horizontal stab. An attitude sensor switched which RADALT pair were active at any given time to preclude a sidewards proximity, like a canyon wall, creating a pull-up warning.

 

'666 prior to GPWS installation

 

After. GPWS antennas at the base of the tail.

 

You can just make out the GPWS antennas beneath the horizontal stab

 

GPWS evaluation essentially consisted of throwing the jet at the ground in various attitudes and verifying that the "roll-out" and/or "pull-up" warning came on when it was supposed to.

 

 

Not the best camo for the Mojave Desert...

 

Approach to a touch 'n' go at nearby George AFB

 

 

 

Refueling from the 'Business Effort' tanker from the Illinois Air National Guard

 

With test events completed, joining up with a 6512TS 'Vark

 

Return to base. South end of Rogers Dry Lake flooded - not unusual in the winter/spring. Always check the status of the lakebed runways before stepping to the jet!

 

Thanks for looking,

Sven

[Latinbear]

Great pictures and narrative as always and much appreciated. What sort of altitude would the GPWS be set for? Given the amount of time the A10 would spend at low level being thrown around was there a risk that it would be constantly triggered and thereby act as a distraction? It's a bit different but I remember seeing an interview with an airforce pilot - I forget the fast jet type he was flying - about attacking targets in Vietnam. He said that the RWRs were going off so frequently on the attack run that he ended up switching them off as they distracted him from the job in hand and were spoiling his concentration. 

[Old Viper Tester]

1 hour ago, Latinbear said:

What sort of altitude would the GPWS be set for?

We used artificially high altitudes as part of the test program risk mitigation. All test programs have to go to a safety review board to investigate risks and dictate risk mitigation procedures. The altitudes that should be set for operational testing was part of the evaluation and resulting recommendations. Different dive angles and descent rates were tested first to determine safe pull-up buffers using max recovery without encountering an accelerated stall. Then roll angles were varied to determine yet an additional buffer for the pilot to rollout and then pull.

 

Then there's always the human factors people telling you what kind of reaction time would need to be added into the calculations. It's one thing to be spring-loaded to pull knowing that a warning should be coming, it's another to be taken by "surprise" and transitioning from what ever had your attention before to pulling up. The system was also supposed to use algorithms to take into account rising or falling terrain under the aircraft - a tough nut when you don't have a forward looking terrain radar.

 

Then there is the factor you mentioned, characterized as nuisance warnings. Make the system too cautious and you get too many warnings - and the temptation to turn it off. Make it not cautious enough and the pilot may not get a warning when it is really needed with disastrous results. 

 

The actual "recovered and heading away from the terrain" altitude used out in the field is often dictated by the operating command and sometimes by wing operations office. Our testing went down to 100 ft above the safety floor (usually 1,000 to 2,000 ft AGL depending on attitude and descent rate of the test point).

 

The final testing was with the black box algorithm"optimized", taking into account dive angle, descent rate, roll angle, reaction time, and terrain uncertainty (rising/falling). They were also looking at engine fan spool-up time, but I don't remember if that ever made it into the mix.

 

Sven

[Alpha Delta 210]

Fascinating. Thank you for the images and detailed explanation.

[Latinbear]

51 minutes ago, Old Viper Tester said:

We used artificially high altitudes as part of the test program risk mitigation. All test programs have to go to a safety review board to investigate risks and dictate risk mitigation procedures. The altitudes that should be set for operational testing was part of the evaluation and resulting recommendations. Different dive angles and descent rates were tested first to determine safe pull-up buffers using max recovery without encountering an accelerated stall. Then roll angles were varied to determine yet an additional buffer for the pilot to rollout and then pull.

 

Then there's always the human factors people telling you what kind of reaction time would need to be added into the calculations. It's one thing to be spring-loaded to pull knowing that a warning should be coming, it's another to be taken by "surprise" and transitioning from what ever had your attention before to pulling up. The system was also supposed to use algorithms to take into account rising or falling terrain under the aircraft - a tough nut when you don't have a forward looking terrain radar.

 

Then there is the factor you mentioned, characterized as nuisance warnings. Make the system too cautious and you get too many warnings - and the temptation to turn it off. Make it not cautious enough and the pilot may not get a warning when it is really needed with disastrous results. 

 

The actual "recovered and heading away from the terrain" altitude used out in the field is often dictated by the operating command and sometimes by wing operations office. Our testing went down to 100 ft above the safety floor (usually 1,000 to 2,000 ft AGL depending on attitude and descent rate of the test point).

 

The final testing was with the black box algorithm"optimized", taking into account dive angle, descent rate, roll angle, reaction time, and terrain uncertainty (rising/falling). They were also looking at engine fan spool-up time, but I don't remember if that ever made it into the mix.

 

Sven

 

Superb! Thanks very much for the informative reply.

[F-32]

Love these shots

[Robert]

Another great selection of photos and background stories. 

 

O/T do you have any photos when the N/AW-10 was at Edwards ?

 

Robert

[Old Viper Tester]

3 hours ago, Robert said:

O/T do you have any photos when the N/AW-10 was at Edwards ?

I'll put up a separate post with what I have. Many are USAF images that were left in the A-10 CTF when it was shut down.

 

See here:

 

Sven

Edited March 25, 2018 by Old Viper Tester
Link added.