Lubrication system

1. A lubrication system for a gas turbine engine, the system comprising:

a planetary gearbox, the planetary gearbox comprising

a carrier,

an oil channel, and

a sump;

an oil tank;

a primary gearbox lubrication system including a primary oil jet in fluid communication with the oil channel of the planetary gearbox via a first opening, the primary gearbox lubrication system configured to pump oil from the oil tank to lubricate the gearbox with a gearbox primary feed;

a secondary gearbox lubrication system including a secondary oil channel disposed inside of the carrier, the secondary oil channel in fluid communication with the oil channel of the planetary gearbox via a second opening at a first end of the secondary oil channel, configured to lubricate the gearbox with oil from the sump when the oil level in the sump reaches a predetermined level;

a crossover flow valve upstream of the gearbox; and

a scoop attached to the secondary oil channel at a second end of the secondary oil channel, the scoop attached to a rotating part of the gearbox,

wherein the crossover flow valve is configured to increase the oil level in the sump to at least the predetermined level in response to a failure of the primary gearbox lubrication system.

2. The lubrication system of claim 1 , further comprising: a gearbox primary feed pump configured to pressurise the gearbox primary feed with oil from the oil tank; and a gearbox scavenge pump configured to pump oil from the gearbox sump to the oil tank along a scavenge flow.

3. The lubrication system of claim 2 , further comprising a gearbox scavenge valve that has an open position, in which oil flows through a gearbox scavenge path to return to the oil tank, and a closed position, in which oil is blocked from returning to the oil tank from the gearbox through the gearbox scavenge path, wherein the gearbox scavenge valve is configured to close in response to the pressure of the gearbox primary feed falling below a predetermined level.

4. The lubrication system of claim 3 , further comprising a gearbox scavenge control valve with an open position in which the gearbox scavenge control valve communicates oil pressure from the gearbox primary feed to a control port of the gearbox scavenge valve and a closed position in which the gearbox scavenge control valve blocks oil pressure from being communicated from the gearbox primary feed to the control port of the gearbox scavenge valve.

5. The lubrication system of claim 4 , wherein the gearbox scavenge control valve is operable to switch between the open and closed positions in response to an electronic signal.

6. The lubrication system of claim 1 , further comprising a turbomachinery lubrication system configured to lubricate turbomachinery of the gas turbine engine with a turbomachinery feed; wherein the lubrication system is configured to supply a crossover flow from the turbomachinery feed to increase the oil level in the sump in response to a failure of the primary gearbox lubrication system, wherein a portion of the turbomachinery feed is diverted to the sump in response to a failure of the primary gearbox lubrication system.

7. The lubrication system of claim 6 , further comprising: a turbomachinery feed pump configured to pressurise the turbomachinery feed with oil from the oil tank and a turbomachinery scavenge pump configured to pump oil from the turbomachinery to the oil tank along a turbomachinery scavenge flow.

8. The lubrication system of claim 6 , further comprising the crossover flow valve configured to block crossover flow from the turbomachinery feed to the gearbox in a closed position, and to communicate the crossover flow from the turbomachinery feed to the gearbox in an open position; wherein the crossover valve is configured to open in response to the pressure of the gearbox primary feed falling below a predetermined level.

9. The lubrication system of claim 1 , wherein the planetary gearbox, further comprises a ring gear, a sun gear and at least two planet gears supported for rotation by planet gear bearings.

10. The lubrication system of claim 9 , wherein the oil channel of the planetary gearbox is configured to receive gearbox primary oil feed and a secondary oil feed from the secondary oil channel at a first location, and to provide oil to a planet gear bearing at a second location, wherein the second location is radially further from an axis of rotation of the carrier than the first location.

11. A gas turbine engine for an aircraft comprising:

an engine core comprising a turbine, a compressor, and a core shaft connecting the turbine to the compressor;

a fan located upstream of the engine core, the fan comprising a plurality of fan blades;

a lubrication system according to any preceding claim, wherein the gearbox receives an input from the core shaft and outputs drive to the fan so as to drive the fan at a lower rotational speed than the core shaft.

12. The gas turbine engine of claim 11 , wherein:

the turbine is a first turbine, the compressor is a first compressor, and the core shaft is a first core shaft;

the engine core further comprises a second turbine, a second compressor, and a second core shaft connecting the second turbine to the second compressor; and

the second turbine, second compressor, and second core shaft are arranged to rotate at a higher rotational speed than the first core shaft.

13. The lubrication system of claim 1 , wherein the scoop is configured to collect oil from the sump of the gearbox when the oil level in the sump is deep enough for the scoop to reach the oil.

14. A method of lubricating a planetary gearbox of a gas turbine engine, the method comprising:

lubricating a journal bearing of the planetary gearbox with oil from a primary gearbox oil feed via an oil channel, wherein the primary gearbox oil feed is provided to the oil channel by a primary oil jet, the oil channel in fluid communication with the primary oil jet via a first opening;

in response to failure of the primary gearbox oil feed, causing an oil level in a sump of the planetary gearbox to increase; and

lubricating the journal bearing of the planetary gearbox with oil from the sump when the level has reached or exceeded a predetermined level, the oil from the sump fed to the journal bearing via a secondary oil channel in fluid communication with the oil channel by a second opening, the secondary oil channel disposed in a carrier of the planetary gearbox, wherein a scoop in direct fluid communication with the secondary oil channel feeds the oil from the sump to the secondary oil channel, wherein the scoop is attached to a rotating part of the gearbox and collects the oil from the sump.

15. The method of claim 14 , wherein lubricating the planetary gearbox comprises splash feeding the oil onto the journal bearing of the planetary gearbox.

16. The method of claim 14 , wherein causing an oil level in the sump to increase comprises operating at least one valve to reduce an oil scavenge flow from the planetary gearbox and/or to provide an additional oil flow to the planetary gearbox.

17. The method of claim 16 , wherein the additional oil flow to the gearbox originates from an oil pump that does not pressurize the primary gearbox oil feed.

18. The method of claim 14 , wherein causing an oil level in the sump to increase comprises diverting oil from a turbomachinery lubrication system to the planetary gearbox.

19. The method of claim 14 , comprising using a lubrication system to perform the method, wherein the system comprises:

the planetary gearbox, the planetary gearbox comprising the sump;

an oil tank;

a primary gearbox lubrication system configured to pump the oil from the oil tank to lubricate the planetary gearbox with the primary gearbox oil feed;

a secondary gearbox lubrication system, wherein the secondary gearbox lubrication system lubricates the journal bearing of the planetary gearbox with the oil from the sump when the oil level in the sump reaches the predetermined level; and

a crossover flow valve upstream of the gearbox,

wherein the crossover flow valve causes the oil level in the sump to increase to at least the predetermined level in response to a failure of the primary gearbox lubrication system.