Is there no stopping International Consolidated Airlines Group (LSE: IAG) shares? It doesn’t look like it.

The British Airways owner is up another 8% in the last week. It’s up 22% over one month, 64% over three months and a massive 145% over the year.

It may be an airline stock, but it’s behaving like a space rocket. No pilot would allow a passenger plane to climb at this speed.

Which poses a problem for investors like me. Momentum stocks always do. There’s a risk that I hop on board, just as they stall. Then drop.

This is the first FTSE 100 stock I want to buy

This has happened to me a lot lately. Even FTSE 100 defence manufacturer BAE Systems, which I once described as the ultimate no-brainer plunged days after I added it to my portfolio last March. I’m still down 10%. In today’s dreadfully warlike world, nobody should lose money on BAE. I have. On paper.

I’m almost too ashamed to admit I bought Nvidia shares for the first time on Friday 17 January. On Monday 27 January it suffered the single biggest stock market loss in US history, falling $600bn as DeepSeek threw down its cut-price gauntlet. There’s no hope for me.

And I’m not going to share how I fluffed the Rolls-Royce growth miracle.

Despite those dire omens, I still want to buy IAG today. I think there’s plenty of fuel still in the tank. The IAG share price still looks good value with a price-to-earnings (P/E) ratio of just 8.6 times.

Yes I know that’s roughly double the P/E of three or four times it traded at a year ago. But it’s still roughly half the FTSE 100 average of 15 times. Which isn’t bad given that it’s the highest flyer on the index over the past year.

I think there’s more growth to come

The early rapid growth stage is over. I’ve missed that, I accept it. Pandemic lockdown hell is now a fading memory. Although it has left IAG with roughly €6bn of debt. That will take a few more years to whittle away.

Airlines are highly sensitive to economic conditions. If a recession hits, demand for air travel could plummet, hurting revenues and profitability. We’re waiting to see how Donald Trump’s mooted tariffs could hit business growth – and their transatlantic travel plans.

Fuel price volatility is a constant concern. Carbon taxes and emissions targets could drive up operating costs. IAG faces a tough balancing act between improving service quality, especially at BA, while competing with budget carriers on price.

Yet I can’t argue with its momentum. Only one thing is holding me back, and no, it isn’t my experience with BAE, Nvidia and Rolls-Royce. I just haven’t got any cash in my trading account.

So I have a second decision to make. Which stock to sell? With the FTSE 100 breaking new all-time highs, I don’t want to ditch anything. Although I’ve got my doubts about spirits giant Diageo…