Start up engines at high altitude

[marpilot]

Hello,

I was trying to start engines at 11000ft (SPJJ) and couldn't introduce fuel as usually - N2 wouldn/t go more than 20.5 %.

Is it a bug, limitation of B733 or requires special procedure?

 

[marpilot]

OK, connecting Ground Air Supply helps in this case

SOLVED

[mfor]

Well the procedure according to the FCOM is

High Altitude Airport Engine Start (above 8400 feet)

For airplanes certified for operation at high altitude airports, accomplish the following:

Ignition select switch ........................................................................Both

Engine start ........................................................................... Accomplish

• The minimum start pressure is 30 psi minus .5 psi for each 1000 feet above sea level.
• An indication of N1 rotation plus a minimum of 20% N2 are required prior to introducing fuel to the engine.
• Engine acceleration will be much slower than during starts at lower altitudes.
• The engine start switch must be held to the GRD position until N2 RPM reaches 50%.

CAUTION: Do not attempt to re-engage the starter above 20% N2 if it is inadvertently allowed to cut out at 46% N2. Note: Fuel fogging from the engine exhaust may occur during a normal high altitude airport start.

It also mentions the following limits

With APU bleed + electrical load, maximum altitude is 10,000 ft.

With APU bleed, maximum altitude is 17,000 ft.

With APU electrical load, maximum altitude is 35,000

 

At SPJJ on ground power I get about 33psi giving me a maximum 21.4% N2 (and 0.4%N1), which should allow an engine start.

It might be possible that aircrafts certified for operation at high airports use different N2 thresholds for fuel injection, although when moving the lever to idle a minimal FF is indicated.

The IXEG 737 seems to inject fuel at 22.4% N2 (doing cross engine bleed start).