Source: Gerben’s Bridge Blog Wednesday, September 27, 2006 
So when do you underlead an Ace in a trump contract? The first rule you learn is: Never!
Here are some things that could go wrong:
* An opponent could win a trick with the King he would otherwise not get.
* Even if partner has the King, this might not gain a trick.
* Partner has the King and it would gain a trick, if only he knew to put it in. He expects declarer to have the Ace and will play accordingly!
* Partner has the Queen, the King is in dummy and declarer plays small and STILL it won’t work, for example:
| K 10 3 | ||
| A 8 7 5 | Q 9 4 | |
| J 6 2 |
On the lead of the 5, dummy plays the 3. Now East will correctly put in the 9!
This was just a short list of horrible things waiting to happen to someone who underleads an Ace. But… as always there are exceptions.
Against a slam with a void
| (1) |
2 |
(3) |
3 |
| (4NT*) | Pass | (5 – 1 KC) |
Pass |
| (6) |
All Pass |
You’re on lead with: AJT743 432 8432 –
Now the correct lead is the 3! We are likely to make exactly 1 Spade trick (declarer is not void because of Blackwood) so a ruff seems like the only possible way to beat it. We must hope partner has the King and then returns a Club to beat the contract.
Their contract will likely make and declarer has no control in the suit.
The opponents bid unopposed:
| 1 |
2 (GF) |
| 3 (extras) |
4 (non-serious cuebid) |
| 4 |
Pass |
Now if you know that they would cuebid a singleton in opener’s suit you may consider leading a or away from Axx. This is safer than usual since partner will know you have the Ace, but declarer will not. The lead away from A loses some of its attraction if declarer can have a singleton Spade though.