Video - Unstable air pressure - startup

[andrej]

Duct pressure fluctuates from 36 to 15 and back to 36. I have three questions, does this happen often? What is the cause of such fluctuation? And any idea why fuel tank would be turned off (MEL?)?

link to the video: https://www.instagram.com/reel/DaNwt5IIlUP/?utm_source=ig_web_copy_link&igsh=MzRlODBiNWFlZA==

Thanks!

[Mariano]

On the 767, once the SAV (starter air valve) opens, the pressure drops noticeably due to the starter demanding a considerable air mass flow rate (APU is already at 101% speed and load control valve is 100% open, and neither can do anything else in order to satisfy the very hungry starter) and, to a much, much lesser extent, because of the new area of pneumatic lines (downstream of the now open SAV) that the air must fill, stabilizing again shortly after. It does not drop again until the second engine start.

Best regards,

Mariano

[andrej]

Thank you, Mariano.

[Mariano]

Andre,

Quick video of duct pressure drop on the 767 (CF6-80C2B6F engines).

Keep an eye on the round DUCT PRESS gauge on the right side of the screen as the starter valve is engaged.

This was at 9 feet MSL at 28C.

https://1drv.ms/v/c/da890b87be2c6bc4/IQDohAH2x0GFQJGXSGrUj6JpAUpSplkPoZp6PLFaTcIWXv4

The starter has to drive its own reduction gear train, its own turbine wheel, the accessory gearbox gear train and all the attached accessories, the transfer gearbox, and the N2 spool. The conversion of air mass flow rate to torque in order to spool up and drive all of the above takes considerable effort.

Best regards,

Mariano

[andrej]

Thank you, Mariano.

I have some follow up questions. Did the DUCT PRESS kept up, once stabilized? Or it was unstable throughout? How do you treat it as a crew? I presume that the worst case is that you are not able to start an engine or are there other threats?

Best,

[Mariano]

Andre,

It always stabilizes, just like it did in the video.

The only procedure we must follow (in the 767) is to discontinue the start (put the start selector back to AUTO - equivalent to pushing in the start selector switch in the 744) if, with the start valve open, the duct pressure does not recover and stays below 21 PSI.

Best regards,

Mariano

[United__744]

Blow through a straw while blocking the end, then release your finger. Same blowing force; lower pressure in the tube.