Hot Starts

There have been numerous Hot Starts with the Mustang. Part of these are due to pilot error and inexperience. However, there are some design issues that Cessna may address to reduce the likelihood in the future.

 

Numerous in this case means around 10 out of a fleet of 250 as of this writing. This is a large amount proportionately to other jet fleets, possibly related to operator experience, but it is not like it occurs to every Mustang frequently either.

 

Why do Hot Starts occur?

Hot Starts occur when there is insufficient rotation of the high speed (N₂) spool during start to get a clean self sustaining ignition. Burning hot gases are not pushed out the back of the jet. The heat in the combustion chamber jumps very quickly and causes damage inside the engine. A single hot start can cost thousands of dollars to inspect and more to repair.

 

If ITT temperature limits are exceeded for just a few seconds, the engine can be easily destroyed.

 

Causes in the Mustang

There are two potential design causes in the Mustang. The Mustang has a smallish battery for its size. In contrast, the Phenom 100, which uses a slightly bigger PW617F (vs the Mustang PW615F), has two batteries - one for avionics and systems, and another just for starting. The single Mustang battery must power the G1000, the beacon, etc while the starter may demand up to 1000 Amps transiently. Cessna recently reduced the battery inspection time from 6 months to 3 months in order to ensure that batteries were in good shape in the field. Cessna is also looking into an alternative battery with more capacity. Even Li-Ion is being considered.

 

The second potential cause occurs during a cross gen start. Engine 1 is up and running and will be used to start engine 2. In some cases, engine 1's generator can fall offline during engine 2's start. This will leave the entire engine 2 starting load on the battery, which likely has not yet fully recovered from starting engine 1. Cessna has changed the starting procedure to increase the running engine's N₂ to 10 percentage points above idle in order to have greater generator output during the start. Since this simple procedure was changed, Hot Starts have been reduced significantly. Cessna is looking into changing the Generator Control Unit and possibly the Starter Control Unit.

 

The Mustang also uses an older version of the P&WC FADEC which has less hot start avoidance logic. The newer FADEC will likely be available in 2010 on the Mustang. It is already available on the Phenom 100.

 

Standard Starting Procedure

 

Here is the standard starting procedure from Cessna:

 

1.	ENGINE START Button	PRESS MOMENTARILY (verify button illuminates)
2.	THROTTLE	IDLE
3.	Engine Instruments	MONITOR
	a. N1	Abort start if no N1 by 40% N2
	b. ITT	CHECK FOR RISE
	Abort start if ITT rapidly approaches 830 dC or shows no rise within 10 seconds. Do not exceed 830 dC for more than 5 seconds, limit 862 dC
	c. Oil Pressure	STEADY INCREASE
	d.  Engine must reach stabilized idle within 45 seconds
	e. N2 display digits will change from WHITE to GREEN when FADEC start sequence in completed.
4.	Engine Instruments	CHECK NORMAL (Battery Current less than 100 amps)
5.	Operating Engine N2	INCREASE to 10 percentage points above ground idle (for a cross-generator start)
6.	Other Engine	START; repeat steps 1 through 4
7.	Ground Power Unit	DISCONNECT (if applicable)
8.	L/R GEN Switches	GEN (if ground power was used)
9.	AVN PWR Switch	ON

 

 

Improving the Standard Procedure

 

Here are some ways to reduce significantly the risk of a Hot Start.

1. Always start into the wind. Forget about the AFM limitations of a 10 kt tailwind as adequate for a start. Turn the plane around. On a quick turn always park in the wind. 
2. Use a GPU. Even if it takes 10 extra minutes or the line guy grumbles. Battery starts are fine for a quick turn, or when the battery was just charged, but not recommended for the first flight of the day.
3. Increase Operating Engine N₂ to Idle + 10%, not 55%. Normal ground idle at sea level is 48.6%. Under different atmospheric conditions, ground idle may be higher, like 52%. Do not use a fixed number of 55% - instead increase N₂ by 10 percentage points, e.g. from 48.6% to 58.6%. The Mustang Checklist Rev 7 lists a fixed 55% but that has been amended to the procedure described here by a Temporary Change (pending the next Rev) dated 29 May 2009.
4. Watch for CAS Messages. If you get a GEN OFF message, abort the start. Your running engine's Gen may have just gone offline, so your battery will fail and your starting engine may get a hot start. Note that the Standard Procedure and Flight Safety training do not talk about monitoring the CAS during a cross-gen start. You cannot rely on the amperage indicator of the running engine - transient spikes like 1000 amps are filtered and will not appear on the engine indicators. Similarly, if you get a FADEC ENGINE CTRL SYS L-R message, abort immediately.
5. Screen flicker or loss of engine instruments means abort. If the G1000 screens flicker or go off, or even just the engine instruments go offline ("Red X's") you are not getting adequate amperage. Abort the start immediately.
6. Use 750 dC as a cutoff. If ITT is rapidly approaching 750 dC, abort. Do not wait for 830. There is a very narrow yellow band on the PW600 engines. By the time you react, it may hit 830 anyway.
7. Wait for 50 Amps on the battery before starting the second engine. The Standard Procedure states 100 amps. This means that the working gen is working really hard to charge the battery (which hasn't recovered yet), and it is asked to put out 1000 Amps - no wonder it may go offline! Let the running engine stabilize, let the battery recharge, and then start the second engine. It should only take 30 seconds or so.
8. Consider using NiCad batteries if GPUs are not readily available. NiCad batteries can put out greater amperage than lead acid. However, they can over temp. If GPUs are not readily available in the places you fly, consider switching to a NiCad as the risk of a battery overtemp is likely lower than a hot start. 
9. Disconnect the battery at the end of the day. The Standard Checklist mentions putting in the gust lock, but not disconnecting the battery. This is not addressed in Flight Safety training. The Mustang has a quick disconnect battery connector. Use it.

 

 

Revised Starting Procedure

 

Changes are in yellow.

 

1.	Aircraft	Parked into wind
2.	PFD TMR-REF	START (to monitor 5b and 5f)
3.	ENGINE START Button	PRESS MOMENTARILY (verify button illuminates)
4.	THROTTLE	IDLE
5.	Engine Instruments and CAS	MONITOR
	a. N1	Abort start if no N1 by 40% N2
	b. ITT	CHECK FOR RISE
	Abort start if ITT rapidly approaches 750 dC or shows no rise within 10 seconds. Do not exceed 830 dC for more than 5 seconds, limit 862 dC
	c. Operating Generator	ABORT if offline (GEN OFF L-R CAS Message for a cross-generator start)
	d. Engine Control System	Amber ENG CTRL L-R - ABORT
	e. Oil Pressure	STEADY INCREASE
	f.  Engine must reach stabilized idle within 45 seconds
	g. N2 display digits will change from WHITE to GREEN when FADEC start sequence in completed.
6.	Engine Instruments	CHECK NORMAL (Battery Current less than 50 amps)
7.	Operating Engine N2	INCREASE by 10% (e.g. 48.6% to 58.6%) (for a cross-generator start)
8.	Other Engine	START; repeat steps 1 through 5
9.	Ground Power Unit	DISCONNECT (if applicable)
10.	L/R GEN Switches	GEN (if ground power was used)
11.	AVN PWR Switch	ON