Erwin: Yes, it can get down to -35F on the ramp in winter, but it's usually not that cold. Mine has the capacity to do series starts like yours but I usually wait a while before starting the second engine until the load is below 100a. And doesn't the Merlin allow for cross-generator start for the second engine (or maybe not on series starts)? The MU-2 does not allow for that. The King Air does it automatically but can't do (and doesn't need) series starts since it only has one 24v battery anyway). The free turbines don't need to move the gearbox and prop when they first spool up so they have very little load.

Michael: Generally speaking, using 48v to start the engines is rather hard on most of the electrical components as well as the starter/generator. When I spoke with Honeywell after purchasing my plane, they strongly suggested I use series as little as possible. There is no question that they are faster, cooler starts that dwell in the critical harmonic RPM range of the engine for less time. With that in mind, when I first bought the plane I did every start series due to high density altitude and temperature (until talking with MHIA and Honeywell engineers)...now I almost never do series starts and plug in when possible. GPU's are usually not available where I fly.
Series is harder on the batteries. The batteries can cost around $2700 each. They seem to last 5 years or more if you use minders.

The chart doesn't actually give you that information.

Are Series starts bad or good for the batteries and airplane?

I was trained to serial start Garrett’s on my Commanders. One of my A&P’s said “would you rather replace a battery prematurely or an engine?”

Merlin can do cross generator starts. Series is faster and I do almost exclusively series starts. When I’ve timed starts I’ve found my bat series starts to be faster than gpu starts.

You mistake my point.

If you have a non engine abnormal at 1 knot over the "chop speed", do you fly or try to stop?

Even if the runway is long?

What about 10 knots over?

20 knots?

Exactly when in the takeoff sequence can you stop and when can't you for any given runway?

The chart doesn't tell you that. It only tells you that you can stop on a runway of X length which rarely will match the runway you are using.

So the chart says 65 knots and 2500 ft and you wouldn't fly on a 2400 ft runway. You only saved yourself from 1 second of added exposure by doing that (if the abnormal occurs 1 second prior to 65 knots, you can stop on 2400 ft).

You would fly on a 2600 ft runway. You only gained 1 second of extra coverage by doing that (if the abnormal occurs 1 second after 65 knots, you can't stop in 2500 ft, so you are flying anyway).

Seems arbitrary and insignificant as a decision process.

By analogy with multiengine airplanes, the accel/stop chart feels like it conveys some sort of safety standard for an SETP, but it does nothing of the sort in real life. When something goes wrong, every pilot has to decide if they should try and stop on the runway or fly away. The PC-12 chart doesn't answer that question at all for any given runway.

Mike C.

The subject of series versus parallel is an ongoing debate with TPE331 owners.

I was decidedly in the series camp at one time using similar logic expressed above, better for the engine, so I'll buy more batteries.

But over time, I have seen the damage series starts do if used routinely. It isn't just the batteries. It is the series/parallel relays, the battery isolate relays, the starter relays, the starter brushes, and the terminals and wires themselves. There are also subtle negative effects on the rest of the aircraft system when the bus voltage drops so badly under a series start (brown out for everything else on the plane).

I've measured the current during a parallel start (~900 amps peak) and during series starts (over 2000 amps, pegged my meter). That is a lot of arcing and wear on contactors and the start current goes through at least three of them (battery isolate, series/parallel relay, starter relay). Within 3 years of buying the airplane and doing series starts, I've had to replace both battery isolate relays and the series parallel relay due to contact wear, as well as two sets of batteries, and overhaul both starter/generators. After switching to primarily parallel starts for the last 8+ years, no more contactor replacements, batteries last much longer, and no starter/generator overhauls.

Batteries DO get damaged by high peak currents. It is not just how far they are discharged, but peak currents do reduce the life of the plates.

I also talked with Honeywell engine guy about parallel starts. They told me that engine wear is not significantly different between series or parallel starts as long as the temperatures are managed properly. Peak start temperature is a far bigger indicator of start wear than start time duration.

Series starts do have their place, particularly with cold weather starts, and I do do them when required, but I don't use them when I don't need to. Despite the parallel start taking longer, the battery recharge afterward is FAR less, indicating much less depletion of the battery. The start peak temps are not appreciably higher in parallel, just the start is slower.

Best cold weather starting tricks, in preferential order:

1. Use a heated hangar the night before.

2. Install engine block and battery pad heaters and plug them in the night before.

3. Use a GPU, but make sure it is more than 1000 amps and engine or AC line powered, "battery carts" are usually no good.

4. Use series start.

5. In super cold, use abort start preheat procedure.

Abort start preheat is as follows:

In parallel, attempt start of engine but abort at around 25-30% (that's usually about where the start gets slow in the cold, if it doesn't just finish the start). Do this for both engines. Install engine plugs and wait 10 minutes. Remove plugs and do series start. The heat from the abort start will warm up the engine core and make it much easier to start. The battery, assuming it is lead acid, will also be warmed up somewhat from the abort start, so despite using it more, it seems to produce good power on the second start.

Mike C.

Stop.

Andrew

Stop.

Andrew

Stop.

Since Piper doesn't publish an accelerate stop distance, this is what I do. Add the 50' takeoff distance to the 50' land distance.

Mike, you're just being argumentative. The accelerate stop distance isn't a perfect metric. It's not the end all be all. It is a tool. A data point of reference. One of many that a conscientious pilot takes into consideration while preparing to launch. I'd rather consider these distances than not. Of what harm is the information?

The hell you say!

This means you are safer than someone who uses the accel/stop distance chart for a PC-12 since you have more coverage where you can stop on the runway. The PC-12 accel/stop figure comes from using Vr as the decision point which is decidedly on the runway and 1.1 Vs1. The 50 ft numbers comes from being 50 ft in the air and at 1.3 Vs1 at that point.


No harm as long as you realize it is an arbitrary decision point among many you could otherwise choose. People who use it as a go, no go criteria for a runway are saying they are willing to choose running off the runway or flying with an abnormal condition once they get past Vr.

Accel/stop for multiengine airplanes is an entirely different thing and should not be confused with the accel/stop numbers Pilatus publishes.

Mike C.

The hell you say!

Ha, ha. Yes, joined a long time ago, but didn't spend much time here until about 6 months ago. Still learning the personalities.