Bleed trip off when start descent classic

[tangosierra]

On 734, when initate descent at high FL (above FL220), after TL set to idle, engine 1 bleed trip off appears, usually resetable immediately. Fault is not on each flight, but more than 50 percent flight affected. Unable simulate fault on ground(in AIR mode).No anomalies during ground run in pneu parameters.Trubleshooting performed, except replacement of precoler . Till now replaced or swaped : PRSOV and regulator, high stage valve and regulator, 490F sw, both thermostats swaped btwn. engines, precooler control valve replaced,pack valve swaped.Leak check on control lines performed sat. Everythink look good.
Do you have experience with this problem? Can be precooler fail (decreased heat exchange)? Upstream side of coling air checked and found OK.
Thanks for help!

[sjap]

Was the precooler control valve replaced too? Try that or swap it.
If on top of descent trip off light comes on, it could also be the HP valve and/or HP regulator. Did you also check if the HP valve is not leaking internally?
The bleed air check was within limits? See form on
http://www.sjap.nl/invul%20Bleed%20PG.xls
Best regards
sjap

[tangosierra]

Hi Sjap,
thanks for recommendation, bleed trip off problem fixed by precooler replacement . On bleed air upstream side of precooler found "impact" damage, probably blocking airflow partially.
Best regards
TS

[hakani]

We had same problem our 734 type a/c, which returned from C check and engine #1 also changed. Technicians at the hangar in Romania who replaced the engine, they wrongly fix the 450F thermostat line between precooler cont. valve 390F thermostat line... It is reaaly interesting because the line has same size (11/16 I remember) There is no murphy:):)

[hakani]

opps I answer the wrong post sorry...

[hakani]

[TMA737]

Hi, very often when we have a pneumatic problem with PACK TRIP OFF involved the precooler and/or preccoler control valve are the last two we usually incriminate, but in according my experience very often this problem has been fixed by troubleshoot those elements
Best regards

[jimm1909]

[73mc]

Probably 90 percent of the time when the w/u states there is a bleed trip when power is reduced for descent look at the precooler control valve first. What happens is the precooler control valve sits near the top of the engine and gets all the heat. It has by far the highest failure rate of all the pneumatic parts on the engine. When the flight crews reduce power for descent 5th stage air is no longer enough to power the pneumatics. At this point the high stage valve opens allowing 9th stage air in. This ninth stage air is much hotter than 5th stage so a degraded precool control valve has a hard time keeping up with all this hot air and bingo....a bleed trip.

[737Inst]

Keep in mind that a bleed trip can only be caused by an overpressure or an overtemperature. On the classic, an overpressure occurs at 180 psi while on the NG it will happen at 220 PSI. Overtempature on both is at 490 F. Since the 5th stage of HP compression is a maximum of about 160 psi (standard day at sea level) ANY true overpressure trip can only be caused by a high stage issue, the valve or the regulator.

In the case stated by the original post, the trips were happening at or above FL220. At that altitude, the 9th stage of HP compression will be less than 180 psi (it is about 150 psi according to engine charts) so at that altitude it is not possible to have a pressure trip - it can ONLY be a temperature trip. To take that one step further, remember that the poster stated that the trip was at top of descent - the thrust levers are at idle - this has to be temperature related.

As the poster above stated, this often is caused by the precooler control valve not functioning quite as well as we might like but one further item may be of concern. The precooler itself may be dirty or partially blocked leading to an inefficiency in the cooling process. Use an inspection mirror and light at the inlet on the 12 o'clock strut and look back through the open precooler control valve (it springloads to the open position) and have a look at the face of the precooler itself to dirt build-up and/or bird parts.

The biggest issue in troubleshooting the pneumatic system is in not identifying the type of trip (temp or press) and changing parts unrelated to the root cause - ie; hp valve or regulator for a temp issue.