Engine surge oscillation at taxi

[Airhunter]

9 hours ago, Machalot said:

@IronMike

I could not reproduce the oscillations by putting the throttle near the detent.

 

However, tonight I had a recurrence of the oscillation with the throttle all the way at minimum in ground idle.  The oscillation had a very regular period of about 1 second, and It was accompanied by a flashing XTANK BRA warning indicator on the left panel during each cycle.  The RPM needle bounced up and down a few hundred RPMs.  If I pushed the throttle forward past the first detent into flight idle, the oscillation and flashing XTANK BRA went away.  It was at the beginning of a ~30 minute multiplayer session, so I have a track file but it is large.  I spent a few minutes fiddling with waypoints and mission planning, but once I started taxiing I noticed the oscillation and stopped to make some observations.  If you would like me to send it to you in a PM, let me know.

 

 

What was the OAT on that MP mission? Field elevation and DA?

[Machalot]

1 hour ago, Airhunter said:

 

What was the OAT on that MP mission? Field elevation and DA?

Hoggit Syria. Near sea level at Adana.  Didn't note the OAT but when I get a chance to reopen the track file I'll look.  I assume OAT means outside air temperature, and DA means density altitude.

[Airhunter]

58 minutes ago, Machalot said:

  I assume OAT means outside air temperature, and DA means density altitude.

Correct.

[Machalot]

10 hours ago, Machalot said:

Hoggit Syria. Near sea level at Adana.  Didn't note the OAT but when I get a chance to reopen the track file I'll look.  I assume OAT means outside air temperature, and DA means density altitude.

Temp 37 C.  QNH 750 mmHg / 29.53 inHg.  QFE at Adana 749.60 mmHg / 29.47 inHg.

[TOViper]

Pretty high temperature. That's why I was initially asking (2nd post) for the OAT.

I remember long time ago that the engine showed oscillations when OAT was high, but currently if you increase the OAT to 50°C the engine dies instantly after spawning into a hot cockpit (considering the throttle position of the real throttle is not synced with the model throttle lever, or it is synced but in the ground idle position). There is a time frame of about ~1 or 2 seconds after spawning for bringing the throttle lever forward to at least flight idle ("flygtomgâng") in order to keep the engine running even at ground.

Seems to be a fuel controller problem, but currently I have no idea if this is happening in the real Viggen or not.

If I'd consider that OATs above 35°C are very seldom in Sweden, there might be a temperature limit for correct/perfect functioning of the fuel controller at these temperatures ...

Edited October 5, 2021 by TOViper

[Machalot]

12 hours ago, TOViper said:

Pretty high temperature. That's why I was initially asking (2nd post) for the OAT.

I remember long time ago that the engine showed oscillations when OAT was high, but currently if you increase the OAT to 50°C the engine dies instantly after spawning into a hot cockpit (considering the throttle position of the real throttle is not synced with the model throttle lever, or it is synced but in the ground idle position). There is a time frame of about ~1 or 2 seconds after spawning for bringing the throttle lever forward to at least flight idle ("flygtomgâng") in order to keep the engine running even at ground.

Seems to be a fuel controller problem, but currently I have no idea if this is happening in the real Viggen or not.

If I'd consider that OATs above 35°C are very seldom in Sweden, there might be a temperature limit for correct functioning of the fuel controller at these temperatures ...

 

The first time I had this issue, months ago, it seemed like a pretty standard day in Caucasus. I didn't take note of the OAT though.