B737 Ng (600/700)

[1.86EPR]

Hi folks,

Can anyone tell me why conditioned air from the left pack, that is used to feed the cockpit, is tapped off upstream of the mix manifold while excess air from the left pack and conditioned air from the right pack both go through the mixing manifold prior to introduction to the cabin sections?

i.e. is there some special reason air destined for the cockpit avoids the mixing manifold?

Thanks!

[jpoth06]

The air supplied to the cockpit is 100% fresh, not mixed with re-circulated air.

[Old Smokey]

Cockpit Air Temperature may be directly controlled by the Temperature Control for the Left Pack. This may result in hotter or colder air than desired for that portion going to the cabin.

Overall cabin temperature control is then achieved by running the Right Pack at such a temperature output as to adjust the Left Pack output Temperature to that desired after mixing.

Fairly standard on most aircraft.

Regards,

Old Smokey

[Mad (Flt) Scientist]

i.e. is there some special reason air destined for the cockpit avoids the mixing manifold?

Almost certainly its a result of the design requirements of ...

§ 25.831 Ventilation.

(a) Under normal operating conditions and in the event of any probable failure conditions of any system which would adversely affect the ventilating air, the ventilation system must be designed to provide a sufficient amount of uncontaminated air to enable the crewmembers to perform their duties without undue discomfort or fatigue and to provide reasonable passenger comfort. For normal operating conditions, the ventilation system must be designed to provide each occupant with an airflow containing at least 0.55 pounds of fresh air per minute.

....

(d) If accumulation of hazardous quantities of smoke in the cockpit area is reasonably probable, smoke evacuation must be readily accomplished, starting with full pressurization and without depressurizing beyond safe limits.

(e) Except as provided in paragraph (f) of this section, means must be provided to enable the occupants of the following compartments and areas to control the temperature and quantity of ventilating air supplied to their compartment or area independently of the temperature and quantity of air supplied to other compartments and areas:

(1) The flight crew compartment.

...

You'll note the subtle difference in paragraph (a) between the crewmembers and passengers, the requirement for smoke evacuation in (d) and the requirement for independent control in (e)(1). Combined, these requirements probably drive that the cockpit supply be made as independent as possible of the cabin supply. (Fire in cabin being one case that comes to mind as being a factor)