Questions Regarding Performance

[Gasman6]

Greetings,

 

I have some questions regarding A-10 cruise performance that perhaps some more knowledgeable members of the community can assist. In list form, they are as follows,

 

1. Is there a list of V-speeds for the A-10 airframe? I am mostly interested in Va, Vd/Vdf, Vf, Vfe, Vle/Vlo, Vs/Vsr, Vx/Vy, and of course, Vne.
2. What is an ideal fuel flow (PPH) for cruise configuration?
3. When do wing and external fuel boost pumps get shut off? Are they shut off? If you only have fuel in the center tanks, do you activate the wing fuel boost pumps at all? It seems, in the sim, that even if your wing tanks are dry, the right engine will not start unless the right main and wing fuel boost pumps are activated. Once they are on, the wing fuel boost pumps may be turned off with no adverse effects, except for a master caution which can be safely extinguished and associated indicator light which may be ignored.

 

Thank you in advance.

Edited January 14, 2023 by Gasman6
Typo (distinguished instead of extinguished, wrong -tinguished)

[ASAP]

7 hours ago, Gasman6 said:

Is there a list of V-speeds for the A-10 airframe? I am mostly interested in Va, Vd/Vdf, Vf, Vfe, Vle/Vlo, Vs/Vsr, Vx/Vy, and of course, Vne.
 

From all the reading I’ve done and guys I’ve talked to the speeds an A-10 pilot cares about are:

Rotate around: 145 knots. (That’s Single engine rate of climb speed or SERC)
best climb: 160
gear and flap limit speed: 200 knots
cruise: generally 250 is usually good but at higher altitudes it’s whatever you can get (usually with throttles in max, yes even with a wingman. Fighter pilots are smart and a wingman can figure it out or communicate with lead if they can’t and then he’ll crack the throttles back a bit)
Vne: 450 knots or Mach 0.75 (whichever is lower)
approach speed: 130 + 2x gas in thousands of pounds (so if I have 3,500 lbs of gas indicated I’d round up to 4 and fly final at 138)
final turn airspeed: 145 + 2x gas in thousands of pounds

[ASAP]

8 hours ago, Gasman6 said:

 

1. What is an ideal fuel flow (PPH) for cruise configuration?
2. When do wing and external fuel boost pumps get shut off? Are they shut off? If you only have fuel in the center tanks, do you activate the wing fuel boost pumps at all? It seems, in the sim, that even if your wing tanks are dry, the right engine will not start unless the right main and wing fuel boost pumps are activated. Once they are on, the wing fuel boost pumps may be turned off with no adverse effects, except for a master caution which can be safely distinguished and associated indicator light which may be ignored.

If your high the ideal fuel flow is whatever you get at max. Once your up at 20k or above the most you’ll get is like 1500 a side anyway. Down lower in the teens 1800-2000 on each engine works well. 
 

Wing and main tanks boost pumps stay on all the time. Unless you have an emergency or a big fuel imbalance. You shouldn’t turn off the wing tanks even when it’s empty. for external tanks: leave them off on the ground. Turn them on at altitude. Shut them off when they are empty. As far as I’m aware the engines take fuel from the internal tanks no matter what. The jet just transfers fuel from the externals into internal tanks every so often to top them off. 

[BuzzU]

Question...........If I land on speed. Doesn't that consider the weight and land at the proper speed?

[Gasman6]

1 hour ago, ASAP said:

If your high the ideal fuel flow is whatever you get at max. Once your up at 20k or above the most you’ll get is like 1500 a side anyway. Down lower in the teens 1800-2000 on each engine works well. 
 

Wing and main tanks boost pumps stay on all the time. Unless you have an emergency or a big fuel imbalance. You shouldn’t turn off the wing tanks even when it’s empty. for external tanks: leave them off on the ground. Turn them on at altitude. Shut them off when they are empty. As far as I’m aware the engines take fuel from the internal tanks no matter what. The jet just transfers fuel from the externals into internal tanks every so often to top them off. 

Thank you for all of your answers, I know some of these speeds will adjust based on weight and balance data, so I appreciate your addition of the calculation formula. Do you know if the fuel boost pump switches are set like this in real life, as well, or is it just a product of the simulation? My understanding, from the common fuel boost pumps used in most general aviation aircraft, that it is never a good idea to run them dry. Indeed, some checklists for some types only have you turn on the boost pumps long enough for engine start, afterwards they can be safely switched off while still guaranteeing a positive flow in the fuel line. Even so, you only activate the pumps for which tanks are carrying fuel, otherwise you can burn out the pump by running it dry. Do you know how or why the A-10 is different?

 

Appreciate your burdening my questions. I realize for some things the answer will have to be "that's just the way it is" and I'll accept that. This is, after all, just for pretend. 

[Yurgon]

11 hours ago, Gasman6 said:

Is there a list of V-speeds for the A-10 airframe? I am mostly interested in Va, Vd/Vdf, Vf, Vfe, Vle/Vlo, Vs/Vsr, Vx/Vy, and of course, Vne.

For a truly comprehensive list, you'll have to dig into the real -1, because the DCS manual doesn't go into all the details on all the instruments and limitations.

However, some of them are perfectly transparent. Flaps and gear speeds (both extended and operating) are all marked by the single yellow line near 200 KIAS on the airspeed indicator. Around that same speed, the flaps will automatically retract to prevent damage, and with the flaps handle in MVR or FULL, they'll come down as the jet slows down below 200ish, 190ish KIAS, however this is a safety measure and pilots should manually retract the flaps well before exceeding 200 KIAS, and only drop them well below 200. No such overspeed protection exists for the landing gear.

Besides memorizing a fixed Vne, you'll find the black and white dashed pointer on the airspeed indicator that gives a dynamic readout of the current maximum airspeed based on atmospheric conditions.

For stall conditions, you can reference the AoA gauge, and in addition you'll get a steady tone in your headset as the aircraft is within 2 units AoA of stalling, and a chopped tone within 1 unit AoA of stalling, see the Stall Warning Control Panel in the DCS A-10C manual.

For approach speeds, you can also reference the AoA indexer on the left side of the HUD frame, but what ASAP wrote about calculating the speeds is of course fully applicable as well.

11 hours ago, Gasman6 said:

What is an ideal fuel flow (PPH) for cruise configuration?

I'm not sure that there is such a thing as an ideal fuel flow in the A-10; however, you have ideal Angles of Attack.

On the AoA indicator you can reference the 15.6 units indication for max range, 17.5 units for maximum endurance and 20 units for approach.

11 hours ago, Gasman6 said:

When do wing and external fuel boost pumps get shut off?

I've never heard nor read of any of them getting switched off by the pilots in flight, unless there was some kind of emergency. That is not to say it isn't done, but I think what ASAP wrote holds true, that pilots simply don't touch these switches.

When the A-10C came out in DCS, the fuel boost pumps defaulted to the OFF position in the cold and dark pit on a ramp start. Based on SME feedback, this was changed to the ON position for all of them because that's how the crew chief will set them up before the pilot steps to the jet. Though TBH, I don't know if the crew chief would keep the wing boost pumps OFF if the jet starts with the wings dry.

11 hours ago, Gasman6 said:

It seems, in the sim, that even if your wing tanks are dry, the right engine will not start unless the right main and wing fuel boost pumps are activated.

Fascinating; is the right wing boost bump actually required to start the right engine with the wings dry? Or does the right main boost pump suffice under these circumstances?

Generally speaking, as far as I'm aware it is correctly modeled in DCS that the left engine can be started without any fuel boost pumps, whereas the right engine does require them (or alternatively it can do a crossfeed-start, if I'm not mistaken).

2 hours ago, BuzzU said:

If I land on speed. Doesn't that consider the weight and land at the proper speed?

Yes, absolutely! But in case you have damage to the pitot static system and when the instruments might be damaged or show erroneous data, a quick calculation of the proper landing speed is a very simple way to fly a good approach without the indexer.

Edited January 13, 2023 by Yurgon
Typos

[Ready]

3 hours ago, ASAP said:

From all the reading I’ve done and guys I’ve talked to the speeds an A-10 pilot cares about are:

Rotate around: 145 knots. (That’s Single engine rate of climb speed or SERC)
best climb: 160
gear and flap limit speed: 200 knots
cruise: generally 250 is usually good but at higher altitudes it’s whatever you can get (usually with throttles in max, yes even with a wingman. Fighter pilots are smart and a wingman can figure it out or communicate with lead if they can’t and then he’ll crack the throttles back a bit)
Vne: 450 knots or Mach 0.75 (whichever is lower)
approach speed: 130 + 2x gas in thousands of pounds (so if I have 3,500 lbs of gas indicated I’d round up to 4 and fly final at 138)
final turn airspeed: 145 + 2x gas in thousands of pounds

What we do to get climb speed is after take usually set ITT 750 and pitch for 200 knots.

Gear down below 200, flaps below 190.

At cruise we usually set an ITT, depends on the location what it is. I am not used to flying full throttle at cruise. 

Here is an example of one of my recent sorties, I created timestamps so you can quickly browse to the desired location. Hope it helps. Please note this is a training sortie where I am learning new things and also screw up here and there.

 

Edited January 13, 2023 by Ready

[BuzzU]

Thanks, Yurgon.

[ASAP]

3 hours ago, BuzzU said:

Question...........If I land on speed. Doesn't that consider the weight and land at the proper speed?

well yes, but... There’s approach speed and then theirs touchdown speed. Most  people interpret that wrong from what I’ve seen. The A-10 is not flown by AOA like swept wing  fighters. What the  AOA indexer is showing you is your touchdown speed. Which is about 10 knots slower than your final approach speed. You don’t get that slow until you start your transition to land. From a the dash one I found on the internet somewhere it says “fly the approach at on speed AOA or calculated landing speed, whichever is higher” unless you are landing immediately after takeoff with a full load of weapons, the AOA will get you slower than you should be. Also if you’re doing that you are probably experiencing an emergency and would fly at single engine approach speeds which are much higher anyway. 
 

I had the opportunity to ask an A-10 pilot I know when and how he uses the AOA indexer. His answer was “we don’t”

[Yurgon]

10 minutes ago, ASAP said:

I had the opportunity to ask an A-10 pilot I know when and how he uses the AOA indexer. His answer was “we don’t”

Fascinating, I wasn't aware of that; thanks for clarifying! 

[ASAP]

2 hours ago, Gasman6 said:

, some checklists for some types only have you turn on the boost pumps long enough for engine start, afterwards they can be safely switched off while still guaranteeing a positive flow in the fuel line. Even so, you only activate the pumps for which tanks are carrying fuel, otherwise you can burn out the pump by running it dry. Do you know how or why the A-10 is different?

Those airplanes aren’t going to be rolling inverted or experiencing high positive/negative g loads, and probably don’t have jet engines. The A-10  engines can run by suction feeding but they are likely to flame out with aggressive maneuvers without positive fuel pressure being continuously provided. 
 

the A-10 fuel system is automated to make pilot workload easier. I’m sure the wing tank pumps probably do shutoff on their own when the tanks run dry or fall below a certain level, but the pilot doesn’t do it.

[ASAP]

1 hour ago, Yurgon said:

For a truly comprehensive list, you'll have to dig into the real -1, because the DCS manual doesn't go into all the details on all the instruments and limitations.

However, some of them are perfectly transparent. Flaps and gear speeds (both extended and operating) are all marked by the single yellow line near 200 KIAS on the airspeed indicator. Around that same speed, the flaps will automatically retract to prevent damage, and with the flaps handle in MVR or FULL, they'll come down as the jet slows down below 200ish, 190ish KIAS, however this is a safety measure and pilots should manually retract the flaps well before exceeding 200 KIAS, and only drop them well below 200. No such overspeed protection exists for the landing gear.

Besides memorizing a fixed Vne, you'll find the black and white dashed pointer on the airspeed indicator that gives a dynamic readout of the current maximum airspeed based on atmospheric conditions.

For stall conditions, you can reference the AoA gauge, and in addition you'll get a steady tone in your headset as the aircraft is within 2 units AoA of stalling, and a chopped tone within 1 unit AoA of stalling, see the Stall Warning Control Panel in the DCS A-10C manual.

For approach speeds, you can also reference the AoA indexer on the left side of the HUD frame, but what ASAP wrote about calculating the speeds is of course fully applicable as well.

I'm not sure that there is such a thing as an ideal fuel flow in the A-10; however, you have ideal Angles of Attack.

On the AoA indicator you can reference the 15.6 units indication for max range, 17.5 units for maximum endurance and 20 units for approach.

I've never heard nor read of any of them getting switched off by the pilots in flight, unless there was some kind of emergency. That is not to say it isn't done, but I think what ASAP wrote holds true, that pilots simply don't touch these switches.

When the A-10C came out in DCS, the fuel boost pumps defaulted to the OFF position in the cold and dark pit on a ramp start. Based on SME feedback, this was changed to the ON position for all of them because that's how the crew chief will set them up before the pilot steps to the jet. Though TBH, I don't know if the crew chief would keep the wing boost pumps OFF if the jet starts with the wings dry.

Fascinating; is the right wing boost bump actually required to start the right engine with the wings dry? Or does the right main boost pump suffice under these circumstances?

Generally speaking, as far as I'm aware it is correctly modeled in DCS that the left engine can be started without any fuel boost pumps, whereas the right engine does require them (or alternatively it can do a crossfeed-start, if I'm not mistaken).

Yes, absolutely! But in case you have damage to the pitot static system and when the instruments might be damaged or show erroneous data, a quick calculation of the proper landing speed is a very simple way to fly a good approach without the indexer.

 

The left and right fuel systems are isolated from each other. To start the right engine you need the boost pumps running other wise there is simply no gas going to the engine. I know the wings feed first but I don’t know if they have to be on to start. I’d suspect the main tank would suffice, but you’d get a nasty fuel imbalance if you left the right wing tank off. 
 

the reason you don’t need the left main boost pump on to start the left engine is because there is a backup DC fuel pump in the left system that is always on and you don’t have a switch for in the cockpit. 

[jaylw314]

1 hour ago, ASAP said:

Those airplanes aren’t going to be rolling inverted or experiencing high positive/negative g loads, and probably don’t have jet engines. The A-10  engines can run by suction feeding but they are likely to flame out with aggressive maneuvers without positive fuel pressure being continuously provided. 
 

the A-10 fuel system is automated to make pilot workload easier. I’m sure the wing tank pumps probably do shutoff on their own when the tanks run dry or fall below a certain level, but the pilot doesn’t do it.

There's a float shutoff switch in the wing tanks when they're empty

[Ready]

3 hours ago, ASAP said:

Those airplanes aren’t going to be rolling inverted or experiencing high positive/negative g loads, and probably don’t have jet engines. The A-10  engines can run by suction feeding but they are likely to flame out with aggressive maneuvers without positive fuel pressure being continuously provided. 
 

the A-10 fuel system is automated to make pilot workload easier. I’m sure the wing tank pumps probably do shutoff on their own when the tanks run dry or fall below a certain level, but the pilot doesn’t do it.

When flying inverted it doesn't take too long for the engines to flame out. There is no procedure (at least I am not aware of it) to adjust fuel pumps before the wing tanks run dry. I know of radio calls to inform the flight that the wings are dry though. 

[ASAP]

2 hours ago, Ready said:

When flying inverted it doesn't take too long for the engines to flame out. There is no procedure (at least I am not aware of it) to adjust fuel pumps before the wing tanks run dry. I know of radio calls to inform the flight that the wings are dry though. 

Yeah there is a limit to how much negative G flight the airplane can tolerate even with the fuel pumps

 
 My understanding is the radio call is basically for the pilots to confirm that the fuel system is function and feeding properly. Around 7 k pounds is when you should have both tanks dry. If one tank still has gas at that point it indicates you could have trapped fuel or a fuel pump issue that requires the pilots attention. They say it over the radio so the pilots can back each other up and provide mutual support. 

[jaylw314]

3 hours ago, Ready said:

When flying inverted it doesn't take too long for the engines to flame out. There is no procedure (at least I am not aware of it) to adjust fuel pumps before the wing tanks run dry. I know of radio calls to inform the flight that the wings are dry though. 

I'm not sure, but I suspect they mainly do that because the wings are NOT self-sealing, they're just integral fuel tanks, unlike the fuselage tanks, so you don't want to get shot at until the wing tanks are empty.

'I don't always get shot at.

But when I do, I prefer empty wing tanks.'

[Gasman6]

Thank you, @ASAP and @Yurgon, you have answered a lot of my questions and pointed me in the right direction. I had ballpark ideas and clues to a few of the reference speeds based on the same cockpit indications as mentioned, but it never hurts to seek verification.

 

9 hours ago, Ready said:

At cruise we usually set an ITT, depends on the location what it is. I am not used to flying full throttle at cruise. 

 

This is curious to me, why is ITT the chosen metric over %RPM or fuel flow? I understand the concept of "flying the engine" from a piston perspective, but for turbofans there is no "leaning" so why would ITT matter from an operational perspective beyond evaluating engine health, and to make sure you never exceed a certain figure determined by the engineers who tested it? As you mentioned, the ITT values will change, but I don't think you meant to say location, perhaps you meant to say OAT or ambient temperature?

[ASAP]

2 hours ago, Gasman6 said:

This is curious to me, why is ITT the chosen metric over %RPM or fuel flow?

Its a more accurate way for aircraft to match up their engines power output. ITT is a better comparison of the amount of thrust the engine is producing. No two engines are tuned exactly the same so two engines at 98% RPM could be producing a slightly different amount of thrust. Two aircraft at 750 ITT will be producing a very similar amount of thrust.

Generally engine cruise settings are set as a function of fuel flow not ITT. By setting a fuel flow every aircraft is burning gas at the same rate so its easy math for everyone involved and someone isn't bingoing out earlier than everyone else. ITT is used for something like an instrument trail departure where it's critical that aircraft keep consistent distance behind eachother as they are flying instrument procedures. Like Ready said, if everyone climbs at a set airspeed and ITT you cannot possibly catch up to the guy in front of you if you are flying the same ground track you should be a pretty much set distance and altitude behind the guy in front of you.

Also "best" cruise airspeed is a bit subjective. If a pilot really wants to gnats ass the cruise setting they'd want to use long range cruise in most cases, unless fuel or time they can stay airborne is an overriding factor. Long range cruise is faster and burns more gas than the max range cruise indicated on your AOA guage. but it gets you to your destination a lot faster for reasonably small increase in fuel consumption. That is calculated with a lot of variables. Weight, altitude, air temperature, etc...  and it's something you have to mission plan prior or look up through a spaghetti chart in the dash-1, which means fighter pilots are probably going to resort to a rule of thumb. From everything I've read, at higher altitudes (>20K) the fuel flow setting for long range cruise is pretty close to where max power is, so if the wingman is able to hang on, I'd just fly at max power.

[Gasman6]

3 hours ago, ASAP said:

Its a more accurate way for aircraft to match up their engines power output. ITT is a better comparison of the amount of thrust the engine is producing. No two engines are tuned exactly the same so two engines at 98% RPM could be producing a slightly different amount of thrust. Two aircraft at 750 ITT will be producing a very similar amount of thrust.

Thank you again, this use of ITT makes more sense and reconciles with my basic understanding of turbofan operation. I had not considered formation instrument departures, nor did I consider that such procedures might exist since formation flight is dangerous enough when you're not in soup. My thinking was a group flight would depart individually, and rejoin once VFR-on-top, or trail at a set distance.

[Ready]

7 hours ago, Gasman6 said:

 

 

 

This is curious to me, why is ITT the chosen metric over %RPM or fuel flow? I understand the concept of "flying the engine" from a piston perspective, but for turbofans there is no "leaning" so why would ITT matter from an operational perspective beyond evaluating engine health, and to make sure you never exceed a certain figure determined by the engineers who tested it? As you mentioned, the ITT values will change, but I don't think you meant to say location, perhaps you meant to say OAT or ambient temperature?

Sorry for not being clear enough. Setting ITT has just been my personal experience during my own training sorties thus far. I looked it up in our fundamentals handbook (476TTP 3-3.A-10C). For cruising to/from AO we set fuel flows to aid in fuel and endurance planning. 

I kindly invite you to check out this comprehensive document, which covers the topics you posted questions about and so much more. 

https://www.476vfightergroup.com/content.php?381-shared-publications

Edited January 14, 2023 by Ready