A gas turbine engine (100; 800; 920) comprising: a system for modulating air flow in the gas turbine engine (100; 800; 920), the system comprising: a seal wall (904; 952; 992) comprising an opening; a seal door (908; 970; 1000) configured to sealably and slideably engage the seal wall (904; 952; 992); and an actuator (505; 555 1-n; 910) configured to move the seal door (908; 970; 1000) over the opening; characterised in that the seal door (908; 970; 1000) is forward of the seal wall (904; 952; 992) and is configured to press against the seal wall (904; 952; 992) in response to a pressure differential between an aft side of the seal door (908; 970; 1000) and a forward side of the seal door (908; 970; 1000) in response to the seal door (908; 970; 1000) covering the opening; and in that the gas turbine engine (100; 800; 920) further comprises: a heat exchanger (335; 928), wherein the seal wall (904; 952; 992) and seal door (908; 970; 1000) are positioned to control air flow across the heat exchanger (335; 928), and the system further comprises a drive element (615; 710) including a worm gear engaged with the seal door (908; 970; 1000) to control the linear position of the seal door (908; 970; 1000) relative to the opening, wherein the actuator (505; 555 1-n; 910) is configured to drive the worm gear to move the seal door (908; 970; 1000). a system for modulating air flow in the gas turbine engine (100; 800; 920), the system comprising: a seal wall (904; 952; 992) comprising an opening; a seal door (908; 970; 1000) configured to sealably and slideably engage the seal wall (904; 952; 992); and an actuator (505; 555 1-n; 910) configured to move the seal door (908; 970; 1000) over the opening; characterised in that the seal door (908; 970; 1000) is forward of the seal wall (904; 952; 992) and is configured to press against the seal wall (904; 952; 992) in response to a pressure differential between an aft side of the seal door (908; 970; 1000) and a forward side of the seal door (908; 970; 1000) in response to the seal door (908; 970; 1000) covering the opening; and in that a seal wall (904; 952; 992) comprising an opening; a seal door (908; 970; 1000) configured to sealably and slideably engage the seal wall (904; 952; 992); and an actuator (505; 555 1-n; 910) configured to move the seal door (908; 970; 1000) over the opening; characterised in that the seal door (908; 970; 1000) is forward of the seal wall (904; 952; 992) and is configured to press against the seal wall (904; 952; 992) in response to a pressure differential between an aft side of the seal door (908; 970; 1000) and a forward side of the seal door (908; 970; 1000) in response to the seal door (908; 970; 1000) covering the opening; and in that the gas turbine engine (100; 800; 920) further comprises: a heat exchanger (335; 928), wherein the seal wall (904; 952; 992) and seal door (908; 970; 1000) are positioned to control air flow across the heat exchanger (335; 928), and the system further comprises a drive element (615; 710) including a worm gear engaged with the seal door (908; 970; 1000) to control the linear position of the seal door (908; 970; 1000) relative to the opening, wherein the actuator (505; 555 1-n; 910) is configured to drive the worm gear to move the seal door (908; 970; 1000). a heat exchanger (335; 928), wherein the seal wall (904; 952; 992) and seal door (908; 970; 1000) are positioned to control air flow across the heat exchanger (335; 928), and the system further comprises a drive element (615; 710) including a worm gear engaged with the seal door (908; 970; 1000) to control the linear position of the seal door (908; 970; 1000) relative to the opening, wherein the actuator (505; 555 1-n; 910) is configured to drive the worm gear to move the seal door (908; 970; 1000).