Suddenly can't land the viggen

[Gunfreak]

Last time I flew the viggen. I landed perfectly in strong crosswind. 

 Noe 7 months later whenever I land the viggen does a pirouette on the landing strip. And often goes of and ends up in the grass. This last time it even flipped and I'm surprised the plane and pilot survived.

 

I land slam the breaks and have activated the basket.

[Graphene]

3 minutes ago, Gunfreak said:

Last time I flew the viggen. I landed perfectly in strong crosswind. 

 Noe 7 months later whenever I land the viggen does a pirouette on the landing strip. And often goes of and ends up in the grass. This last time it even flipped and I'm surprised the plane and pilot survived.

 

I land slam the breaks and have activated the basket.

It would be nice if you demonstrated these pirouettes on video.

[Gunfreak]

2 hours ago, Graphene said:

It would be nice if you demonstrated these pirouettes on video.

 

[Graphene]

42 minutes ago, Gunfreak said:

 

Too hard braking as for me. 

[Gunfreak]

2 hours ago, Graphene said:

Too hard braking as for me. 

I've been told you should just slam the breaks. 

[Graphene]

51 minutes ago, Gunfreak said:

I've been told you should just slam the breaks. 

Many planes in the DCS have too strong brakes. I would say that you need to put the force on the axis (if an axis is used) about 50-60 percent, or even less.

Edited August 23, 2022 by Graphene

[G.J.S]

I don’t own Viggen (yet), but what I have seen in real world is reverse thrust being used down to a guessed approx 50kts, then wheel brakes from there.

Why burn out the brakes from the moment you land?

[razo+r]

You could also, instead of applying rudder, either try applying left/right stick (roll basically, it actuates the rudder aswell but allowing for finer control) or easy off a bit of the brake on one or both sides.

[Jagdgeschwader]

vor 3 Stunden schrieb G.J.S:

Why burn out the brakes from the moment you land?

Just as he said. Don't use the brakes immediately on a touchdown, use the thrustreverser instead and apply more throttle if needed ( the aircraft slows down really fast).

You can use the wheel brakes with the thrust reverser but don't need to stand on the brakes for that, just apply the brakes a little bit and that's it, but don't forget that in the Viggen your main brake at landing is the thrust reverser.

Hope it helps. 

[renhanxue]

On 8/23/2022 at 8:58 PM, G.J.S said:

I don’t own Viggen (yet), but what I have seen in real world is reverse thrust being used down to a guessed approx 50kts, then wheel brakes from there.

Why burn out the brakes from the moment you land?

On modern air shows they're pretty gentle since it's a museum aircraft, so they don't use the wheel brakes that much. The flight manual procedures for the real aircraft also note that in most cases heavy wheel braking is not necessary when using the thrust reverser and short landing procedures. However, it also explicitly permits going absolutely balls to the wall with both thrust reverser and wheel brakes, if the situation so requires (imagine for example a slippery 800m strip with a bomb crater near one end). There are several flight manual pages of caveats attached to this, however. I'll attempt to summarize the section on short runway landing procedures below. Most of this is probably in the DCS flight manual too, but I enjoy nerding out.

Prerequisites

The 15,5° alpha no-flare approach must be used. With a heavy aircraft in warm weather this might not be possible because of restrictions on how much of a thrust margin you need to safely be able to go around from a high alpha/low speed approach.

• Aircraft weight < 14 tons (<80% fuel with a clean aircraft)
• Crosswind component < 30 km/h
• Must not be carrying rb 04, rb 15 or Sidewinders on the wing pylons
• Runway width >= 17 m.

Procedure

• Arm thrust reverser after gear is down and locked (three green lights)
• On final, the aircraft must be trimmed out neutral in pitch
• If using AFK, disengage it when passing the runway threshold
• Reduce throttle to flight idle when the main wheels touch down
• Ensure alignment with runway heading before the nose wheel touches down
• Pull gently back on the stick after the nose wheel touches down; the higher the throttle setting the harder you need to pull
• Increase throttle as required no earlier than 1,5 seconds after nose wheel touches down; however, if the actual engine RPM is at flight idle or lower the throttle may be increased to mil power immediately on nose wheel touchdown
• Wait until two seconds after touchdown before using the wheel brakes
• Reduce throttle to flight idle no later than when the REV AVDR/TRANSONIC light illuminates; the rpm should be at ground idle by the time the aircraft comes to a stop.

Notes and caveats

• By far the greatest danger during a short landing is the risk of losing yaw control. Although the aircraft has good yaw stability under ordinary circumstances, the thrust reverser disturbs the airflow over the tailfin, which significantly degrades the aircraft's yaw stability. These yaw disturbances become increasingly severe with higher speed and higher reverse thrust settings.
• At very high speeds, engaging the thrust reverser degrades yaw stability to an unacceptable degree, even with the throttle at flight idle. Hence, arming the thrust reverser at speeds above the normal 15,5° glideslope approach speed is strictly prohibited, even in an emergency situation.
• Setting the throttle to full reverse (mil power with the reverser engaged) must not be done before the speed has dropped below 250 km/h.
• The autopilot's SPAK mode automatically uses a special yaw damping mode while the thrust reverser is engaged, which helps mitigate the yaw stability issues to a significant extent, but does not eliminate them. If SPAK mode is not available (e.g. due to an autopilot fault condition), the thrust reverser must not be used at speeds above 200 km/h, and once deployed the throttle setting must not exceed flight idle, except in an emergency.
• Do not push the stick forward while reversing at high throttle settings. The resulting reduced weight on the main gear severely degrades the yaw stability, and in extreme cases the main gear may leave the runway entirely.
• Avoid using asymmetric wheel braking if possible; using only the rudder for yaw control yields higher precision.
• In case of abnormal yaw disturbances during thrust reversal, immediately reduce throttle to ground idle.
• Using the thrust reverser on snow or ice may reduce the runway friction coefficient.

Edited June 23 by renhanxue
clarifications

[Machalot]

2 hours ago, renhanxue said:

On final, the aircraft must be trimmed out neutral in pitch

Does this mean trimmed to hold the correct approach alpha, or trim set to zero? 

[renhanxue]

4 hours ago, Machalot said:

Does this mean trimmed to hold the correct approach alpha, or trim set to zero? 

I understand it to mean the former (hold correct approach alpha), but now that you ask I'm actually genuinely uncertain. 

Edited August 24, 2022 by renhanxue

[Sideburns]

18 hours ago, renhanxue said:

On modern air shows they're pretty gentle since it's a museum aircraft, so they don't use the wheel brakes that much. The flight manual procedures for the real aircraft also note that in most cases heavy wheel braking is not necessary when using the thrust reverser and short landing procedures. However, it also explicitly permits going absolutely balls to the wall with both thrust reverser and wheel brakes, if the situation so requires (imagine for example a slippery 800m strip with a bomb crater near one end). There are several flight manual pages of caveats attached to this, however. I'll attempt to summarize the section on short runway landing procedures below. Most of this is probably in the DCS flight manual too, but I enjoy nerding out.

Prerequisites

The 15,5° alpha no-flare approach must be used. With a heavy aircraft in warm weather this might not be possible because of restrictions on how much of a thrust margin you need to safely be able to go around from a high alpha/low speed approach.

• Aircraft weight < 14 tons (<80% fuel with a clean aircraft)
• Crosswind component < 30 km/h
• Must not be carrying rb 04, rb 15 or Sidewinders on the wing pylons
• Runway width >= 17 m.

Procedure

• Arm thrust reverser after gear is down and locked (three green lights)
• On final, the aircraft must be trimmed out neutral in pitch
• If using AFK, disengage it when passing the runway threshold
• Reduce throttle to flight idle when the main wheels touch down
• Ensure alignment with runway heading before the nose wheel touches down
• Pull gently back on the stick after the nose wheel touches down; the higher the throttle setting the harder you need to pull
• Increase throttle as required no earlier than 1,5 seconds after nose wheel touches down; however, if the actual engine RPM is at flight idle or lower the throttle may be increased to mil power immediately on nose wheel touchdown
• Wait until two seconds after touchdown before using the wheel brakes
• Reduce throttle to flight idle no later than when the REV AVDR/TRANSONIC light illuminates; the rpm should be at ground idle by the time the aircraft comes to a stop.

Notes and caveats

• Although the aircraft has good yaw stability under ordinary circumstances, the thrust reverser causes powerful yaw disturbances, which become more severe at high speeds and high thrust settings. To compensate for this, the autopilot's SPAK mode automatically uses a special damping mode when the thrust reverser is active.
• Using the thrust reverser at very high speeds rapidly degrades the yaw stability to a dangerous degree. Hence, using the thrust reverser at speeds above the normal 15,5° glideslope approach speed is strictly prohibited, even in an emergency situation. In any case, do not increase throttle to mil power at speeds above 250 km/h.
• Do not push the stick forward while reversing at high throttle settings. The resulting reduced weight on the main gear severely degrades the yaw stability, and in extreme cases the main gear may leave the runway entirely.
• Avoid using asymmetric wheel braking if possible; using only the rudder for yaw control yields higher precision.
• In case of abnormal yaw disturbances during thrust reversal, immediately reduce throttle to ground idle.
• Using the thrust reverser on snow or ice may reduce the runway friction coefficient.
• If SPAK mode is not available (e.g. due to an autopilot fault condition), the thrust reverser must not be used at speeds above 200 km/h, and once deployed the throttle level must not exceed flight idle, except in an emergency.

 

Amazing, thank you. Truly an encyclopaedic knowledge of the Viggen.

[Graphene]

18 hours ago, renhanxue said:

On modern air shows they're pretty gentle since it's a museum aircraft, so they don't use the wheel brakes that much. The flight manual procedures for the real aircraft also note that in most cases heavy wheel braking is not necessary when using the thrust reverser and short landing procedures. However, it also explicitly permits going absolutely balls to the wall with both thrust reverser and wheel brakes, if the situation so requires (imagine for example a slippery 800m strip with a bomb crater near one end). There are several flight manual pages of caveats attached to this, however. I'll attempt to summarize the section on short runway landing procedures below. Most of this is probably in the DCS flight manual too, but I enjoy nerding out.

Prerequisites

The 15,5° alpha no-flare approach must be used. With a heavy aircraft in warm weather this might not be possible because of restrictions on how much of a thrust margin you need to safely be able to go around from a high alpha/low speed approach.

• Aircraft weight < 14 tons (<80% fuel with a clean aircraft)
• Crosswind component < 30 km/h
• Must not be carrying rb 04, rb 15 or Sidewinders on the wing pylons
• Runway width >= 17 m.

Procedure

• Arm thrust reverser after gear is down and locked (three green lights)
• On final, the aircraft must be trimmed out neutral in pitch
• If using AFK, disengage it when passing the runway threshold
• Reduce throttle to flight idle when the main wheels touch down
• Ensure alignment with runway heading before the nose wheel touches down
• Pull gently back on the stick after the nose wheel touches down; the higher the throttle setting the harder you need to pull
• Increase throttle as required no earlier than 1,5 seconds after nose wheel touches down; however, if the actual engine RPM is at flight idle or lower the throttle may be increased to mil power immediately on nose wheel touchdown
• Wait until two seconds after touchdown before using the wheel brakes
• Reduce throttle to flight idle no later than when the REV AVDR/TRANSONIC light illuminates; the rpm should be at ground idle by the time the aircraft comes to a stop.

Notes and caveats

• Although the aircraft has good yaw stability under ordinary circumstances, the thrust reverser causes powerful yaw disturbances, which become more severe at high speeds and high thrust settings. To compensate for this, the autopilot's SPAK mode automatically uses a special damping mode when the thrust reverser is active.
• Using the thrust reverser at very high speeds rapidly degrades the yaw stability to a dangerous degree. Hence, using the thrust reverser at speeds above the normal 15,5° glideslope approach speed is strictly prohibited, even in an emergency situation. In any case, do not increase throttle to mil power at speeds above 250 km/h.
• Do not push the stick forward while reversing at high throttle settings. The resulting reduced weight on the main gear severely degrades the yaw stability, and in extreme cases the main gear may leave the runway entirely.
• Avoid using asymmetric wheel braking if possible; using only the rudder for yaw control yields higher precision.
• In case of abnormal yaw disturbances during thrust reversal, immediately reduce throttle to ground idle.
• Using the thrust reverser on snow or ice may reduce the runway friction coefficient.
• If SPAK mode is not available (e.g. due to an autopilot fault condition), the thrust reverser must not be used at speeds above 200 km/h, and once deployed the throttle level must not exceed flight idle, except in an emergency.

 

This post should be pinned somewhere in the Viggen thread!) Top!