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Iw18066711
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Joined: Sat Jan 16, 2021 5:31 pm

Boeing 737 Engine Bleed Precooler System Defiency

Sat Jan 16, 2021 5:48 pm

I would like to report an error or deficiency about in the wing anti ice system and engine bleed precooler system of Boeing's old 737 and 737ng's.

There is icing condition restriction for the fully open position of the precooler control valve in MEL. ( 36-03-2B. it may be inoperative fully open position and there is a restriction that aircraft not operated in icing conditoin).

If precooler control valve in fully open position , it may cause to drop to undesirable temperatures of the engine bleed air, that used in wing anti ice system. For the reason , there is icing condition restriction for relevant MEL item that in order to avoid in risk of icing on the wings.

In the 4 scenarios I explained below, the precooler control valve is work in a fully open position. 

1- If the 390F sensor continuously discharge air, before reaching the required temperature. (May be in stuck open position as faulty. Normally it starts to discharge air at 390F).

2 - If the precooler control valve in stuck open position as faulty. (Normally valve is closed. Valve goes open position if discharge air from in the control pressure sense line when 390F sensor heatup, it must be working modulated ).

3 - If the wing thermal anti-icing solenoid valve in stuck open position as faulty. (Normally selenoid valve is open position when the anti-ice system is used on the ground. İt is close position in air).

4- If there is air leak in the control pressure sense line of 390F sensor . (Normally there should be no air leakage in this line. If there is air leaks ,the precooler control valve is work in a fully open position.)

Precooler control valve in fully open position is not noticed during the preflight inspection, nor is this communicated to the cockpit crew via warning lights or other indications. Because  precooler control valve as faulty  fully open position or always working open position not create malfuction in engine bleed system.

This situation can only be noticed , if the system is tested in schedule maintanence.

While there is icing restriction in MEL (36-03-2B) for open position of precooler control valve. Isn't  necessary to must be an indication or warning system in the cockpit and during the pre-filght inspection? Shouldn't it be known that the position of this valve is? 

These situations I explained above is not in contradiction with MEL item ?

If this situation create unsafe , the airplanes should not fly according to the MEL item.

If this situation not create unsafe , these system parts should be diactivated on the airplane.

Because most causing malfunction and replaced parts are the precooler control valve, 390F sensor and wing thermal anti ice solenoid valve in these system. Operators have been spending a lot of money for defective system parts replacement and manhour when any failure in system for long years.

According to my research, these system parts change once a year in each engine. The average cost per airplane is at least 50 thousand dollars. There are approximately 10 thousand of these airplanes in the world. If we calculate, there is a high cost.
 
unimproved
Posts: 258
Joined: Sun Jun 22, 2014 7:14 pm

Re: Boeing 737 Engine Bleed Precooler System Defiency

Sun Jan 17, 2021 8:27 pm

It won't lead to a plane falling from the sky during icing, but it will make the anti ice slightly less effective.

Not having an indication is the result of using an ancient design. Newer models do have the indication (but will only show it when it's in an unexpected position).
 
Iw18066711
Topic Author
Posts: 3
Joined: Sat Jan 16, 2021 5:31 pm

Re: Boeing 737 Engine Bleed Precooler System Defiency

Sun Jan 17, 2021 8:55 pm

if the defective situation is unknown , lower efficiency is acceptable and safe. But if the defective situation is noticed , lower efficiency unacceptable and unsafe, and the airplane must fly with restriction according MEL. Is there no contradiction in this?
 
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fr8mech
Posts: 8167
Joined: Mon Sep 26, 2005 9:00 am

Re: Boeing 737 Engine Bleed Precooler System Defiency

Sun Jan 17, 2021 11:20 pm

Iw18066711 wrote:
if the defective situation is unknown , lower efficiency is acceptable and safe. But if the defective situation is noticed , lower efficiency unacceptable and unsafe, and the airplane must fly with restriction according MEL. Is there no contradiction in this?



Not at all. That’s why we have scheduled maintenance that digs a little deeper into the systems.

Question: how many cracks are there in the wing or fuselage of any given aircraft? None? One, two, five, more? Once identified, usually during a scheduled maintenance check, the crack(s) is repaired or continued as allowed.

Once a discrepancy is identified, it must be addressed.
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battlegroup62
Posts: 62
Joined: Mon Jul 03, 2017 5:05 am

Re: Boeing 737 Engine Bleed Precooler System Defiency

Mon Jan 18, 2021 1:43 am

fr8mech wrote:

Not at all. That’s why we have scheduled maintenance that digs a little deeper into the systems.

Question: how many cracks are there in the wing or fuselage of any given aircraft? None? One, two, five, more? Once identified, usually during a scheduled maintenance check, the crack(s) is repaired or continued as allowed.

Once a discrepancy is identified, it must be addressed.


Precisely, and also why at times you have turn a blind eye as it were, because its not time to start peeling that onion. We do it all for customers because they are aware of wear rates on items, and know it will make it till next time if its just in limits now. Its all about money.
We have to keep planes airworthy. That doesn't mean they have to fly.
 
Iw18066711
Topic Author
Posts: 3
Joined: Sat Jan 16, 2021 5:31 pm

Re: Boeing 737 Engine Bleed Precooler System Defiency

Mon Jan 18, 2021 1:04 pm

The example you gave is about the current technology. It would be used if material that does not crack could be produced with the current technology. But it is impossible with current technology. For this, damage is determined by checking in planned maintenance. Measures can be taken in the incident I have described. For example, a system that shows the temperature can be installed to the wing anti ice system.

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