Why no APU on a TP?

[jau]

Hi,
I was wondering today why a jet aircraft starts its engines using an APU and a Turboprop simply uses the battery. Why is is as they are both turbines at the end of the day?
Cheers
Rich

[AerocatS2A]

The turbo-props I've been in have started from the APU.

[411A]

Hmm, seem to remember that some models of the Fokker F.27 had APU's a long time ago, and certainly many models of the Fairchild built FH227 did as well.....long ago.

[AerocatS2A]

The only turbo-props I've flown in lately have been an Embraer 120 and a Dash 8, both had APUs (and started from them.) A battery start is avoided on the Dash, they use ground power or the APU as a preference.

Obviously smaller turbo-props such as a Kingair or similar wouldn't have an APU for weight and practicality reasons.

[Capt Claret]

Most jet engines are air start, where the APU bleed air is ducted to the engine and provides the motive force to crank the engine for starting. If APU air is not available then an external air source is needed, otherwise there is no start.

All turboprop engines that I am famiar with have DC electric starter motors, hence can start on ship's batteries, or with a ground power cart providing current, or the APU providing the current.

Some jet engines, such as the ALF502/LF507 also have electric starter motors. In this particular instance, 4 engines on a 146 can be started in a shorter time span than 2 engines on 737/717 types.

Again with the 146, BAe in their own inimitable style produced many variants that could not start an engine without either the APU or a DC ground cart. In the fleet I flew, there were two exceptions, both -300s. In these examples, one pilot had to exit his/her seat, open the hatch into the E&E bay, and press a switch that was recessed into the hatch opening. This would then allow an engine to be started on the ship's battery. The hatch would have to be replaced, lest the cabin crew disappear down into the E&E and the pilot would resume their seat.

I kid you not!

[DVR6K]

Believe some turboprops without a stand alone APU are fitted with prop brakes that allow the engine to run without turning the propellers thus providing electrical power on the ground. Presume the brake is merely disconnected to start the prop turning on the live engine and battery start on the other one.

[Clandestino]

ATR-42 300 has two wing mounted APUs that even provide some forward thrust through attached propellers. 500 series has real engines,though.

Now seriously: prop brake seems to be ATR's specialty. Usually it's installed on No2 eng only to use HP spool as APU but not quite. Engine intake is to close to exhaust and even 2kt of tailwind component provides your unlucky pax with some burnt kerosene aroma. Also exhaust is directed at service door, making baggage loading difficult at the very least.

[Rainboe]

Obviously the modern turboprops have small APUs, but there really were no such beasts in the early days of the Friendship, Viscount and the HS748 40 years ago. The HS748 had 4 meaty batteries for DC starter motors on the RR Dart turboprops. Although having a 12 foot, 4 bladed prop, it was startable as long as you didn't waste battery power on the ground. So the technique was to configure the aeroplane for departure on the taxi in, and as you shut down to kill the battery switch and be electrically dead until start up time. The prop brake was purely used to avoid damaging the propellor leading edges with passenger heads- the Dart could not run with the prop braked.

[Piltdown Man]

Most turboprops have relatively small engines which means that electric starters are generally the most appropriate. Therefore, the options available for the designers for a source of electric power are batteries, APU or GPU. In each case, the electric source has to be reliable because its failure may result a hot start and subsequent (expensive) engine damage. But this has to be balanced against weight (batteries and APU are heavy) and availability (a bush strip is unlikely to have a GPU). But I think I would be true in saying that most modern turboprop operators prefer their aircraft to be started by sources in the following order: GPU, APU and then battery. Only in a smaller turboprop would a battery start be the "preferred" option.

Your average jet engine however, is a fairly large beast and so far air (driving an "air motor" connected via an accessory gearbox to one of the shafts of the engine) has been the best method of starting it. Again weight vs reliablity vs cost will have been taken into account. Air is easy to pipe around the place, it's normally readily available from APU's (or ground Air Start units when your APU is broken), installed to supply electricity when the engine driven generators are not running and will allow a consistent, reliable start.

[jau]

Thanks for your replies.
I asked because I was travelling on a saab 340, the aircraft made a quick (30mins?) stop enroute, after both engines were shut down half the cabin lights went out and things went awfully quiet. There was no GPU, yet everything in the cockpit remained on. Just wondered how long you could remain in this configuation for?
I mean, everything has to be 'on' to preflight and check the equipment, but how do you have enough battery power to power the avionics, and still turn both engines? I only fly small things were the engines are started about 2 mins after the battery comes on, then everything is done when the alternator is alive and kicking. Just curious, thanks!

[PAXboy]

but how do you have enough battery power to power the avionics, and still turn both engines?

Non-pilot speaking. You only need enough battery to start one engine!!

[Piltdown Man]

jau - You'll probably find that many of the systems were on in the cockpit - not all. Most aircraft can "function" to varying degrees, on batteries for at least 30 minutes. But generally only the essential/emergency systems are working, everything else is automatically "disabled".

[The Flying Cokeman]

Clandestino
Quote: "Engine intake is to close to exhaust and even 2kt of tailwind component provides your unlucky pax with some burnt kerosene aroma"

Ever considered taking the bleeds off when operating Hotel mode in tailwind

tfcm

[Clandestino]

As GPUs are readilly available on every airport I've flown into, hotel mode is not used for DC power but for air conditionning only. So taking off bleed no2 and leaving eng in hotel mode would be silly, to say the least.

[The Flying Cokeman]

okay,
Ever considered parking nose into wind then, I'm sure your pax would appreciate that
tfcm

[AtoBsafely]

I've flown dash 8s for while...
My previous company had the APUs removed, because the weight penalty wasn't worth the convenience of extended independent power/air conditioned ops. We had GPUs and at all our regular airports, and external air at most.
My current company has to pay a penalty and one extra person to use a GPU - they find it cheaper to use APUs everywhere and accept the reduced load and extra maintenance.
It just comes down to the dollars; that varies a lot with the route structure and type of pax you have, and the union rules you have to work with.
OJ

[punkalouver]

Most turboprops are capable of starting engines with battery power only for the starter. Some of the medium size ones like the Dash-8 may have an APU, but the APU isn't required to start the engine. On the big turboprops such as the C-130 with Allison's, I think you will find that an APU is required because those starters need air to turn them over. Lose the APU and you are not going anywhere without a ground air supply.

Anybody know what is required for the Transall or the Britannia?

[Clandestino]

Ever considered parking nose into wind?

Never had to worry about that as smaller airports are really helpful and always arrange parking into wind, even before being asked, but bigger airports don't have the same flexibility. Days of parking facing east/facing west on MUC srs 300 stands are long gone.

[ferrydude]

And I might add that the reverse could be asked. Plenty of jets do not have APUs installed. Most DC-8s don't, early Lears, Etc.

[Ingwe]

Used to fly the DHC-5 Buffalo (MAUW 45 000lbs). We had air driven starters so hence had an APU also as we where flying in Sudan into VERY short strips with no facilities it was definately needed. Always worried me that if we lost it we where going nowhere. Thankfully it never gave us any problem.