Unlike many other airports, Manchester actively encourages aircraft photography.
I assume that you or someone willing will be driving, so I've posted directions, post codes or both to the below.
All the below locations favour a lens similar to a 70-300mm.
The most well known place for photographing aircraft at Manchester is the Aviation Viewing Park. Unfortunately due to the positioning of the AVP in relation to the airport, aircraft are, for the vast majority of the day, backlit. Parking here is also very expensive, 3 hours will cost you £12.
Post code: WA15 8XQ
Admittedly, I've not got many shots from the Viewing Park to hand at the moment, you'll have to bear with me.
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I'd advise that you'd head to one of the locations on the south side of the airfield, if aircraft are arriving and departing from Runway 23R, then personally I'd favour the field off Holly Lane. The sun's in the right place for most of the day, early morning to around 14.30 when the sun tends to shift round unfavourably.
Post code: SK9 4JL
As the road takes a left hand turn of approximately 90 degrees, there is a stile in the right hand corner. Having hopped over this, follow the well trodden path to a mound of earth about 6 or 7 metres high. At the top of this mound, there's views over the perimeter fence which allow for photographing aircraft on the approach and preparing to take off.

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There are two spots for aircraft departing on the newer runway, and, assuming you're heading from Wilmslow, can be accessed by taking the third exit on the roundabout before the runway tunnels. If you're heading from the M60, this is the first exit on the roundabout after the two tunnels. This is the best place to park, but whatever you do, DO NOT BLOCK THE CRASH GATE.
You have two options now, if aircraft are arriving on Runway 05R, carefully cross the dual carriageway and go up the steps. Note, this place gets boggy, reet boggy. Follow the well trodden path behind the fire station, down the hill, cross the bridge and up the other side. Continue along the path, which is parallel with the runway, for around 20mins until you come across another grey building not too dissimilar to the fire station. I tend to stay in this area for both arrivals and departures.
A couple of sample shots, taken around 300mm:
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Starting from the same location as the last, instead of crossing the dual carriageway, go through the gate on the left hand side of the road and continue along the path until reaching a large area of open grassland. On your left is a pretty steep hill, walk up here until reaching the gate at the top. From here, aircraft around the size of an A330 departing from the far runway will fill the frame relatively easily. Aircraft departing from the closest of the two runways won't have taken off yet, but will be travelling fairly quickly, so your panning needs to be fairly good.
An example of a 737 departing from the far runway at 300mm.
Hopefully this'll be of some use. Oh, and if you get lost, that's not my fault!
Good luck.
Cheers,
Martin