TristarSteve From Sweden, joined Nov 2005, 4160 posts, RR: 33 Posted (8 years 5 months 2 weeks 5 days 22 hours ago) and read 4056 times:
Boeing 737 from 300 onwards have a WAI system that can operate on the ground. It has temp sensors in the leading edge that open and close the valves. When the throttles are advanced, the valves are closed (for performance reasons, WAI uses a lot of air), and when the aircraft lifts off the system closes the valves and trips to OFF.
The B737-200 ( and some early -300s)
all have WAI that is inhibited on the ground, in fact the B747-400 is inhibited with the leading edge flaps not UP.
Not normally needed & to avoid overheat.Remember its WTAI & not De-Icing.
Also the Test sw presnt with Bleeds available can be used to momentary use WTAI on grd until thermal sw activates Valve to close to prevent ovht.
MX757 From United States of America, joined Aug 2005, 628 posts, RR: 11
Reply 2, posted (8 years 5 months 2 weeks 5 days 6 hours ago) and read 3956 times:
Without any airflow going across the wings to keep the leading edges cool not only will they overheat they will start to warp and wrinkle too. I saw this happen once on DC-9-10. We had to replace all the leading edges on both wings! Not a fun job at all!
Troubleshooter From Germany, joined Feb 2005, 423 posts, RR: 4
Reply 4, posted (8 years 5 months 2 weeks 4 days 21 hours ago) and read 3883 times:
Quoting MX757 (Reply 2): Without any airflow going across the wings to keep the leading edges cool not only will they overheat they will start to warp and wrinkle too
Thats why the B737 wing anti-ice system uses overheat switches (one per wing) to control the system on the ground. They close the valves at a set temperature or if the thrust levers are advanced and they open the valves again after temperature drop or thrust reduction.
The switch trips off at lift off due to performance considerations as the bleed demand for wing anti-ice is a lot. By setting the switch manually to on again, the crew takes the decision to use the big amount of air from the engines or not.