El avión iba a despegar a las siete, pero al final despegó con un poco de retraso.

Iba a + infinitive is a very common way to say was going to + verb in Spanish.

So:

• iba a despegar = was going to take off

It often suggests a plan, an expectation, or something that was about to happen.

By contrast:

• despegaba can also refer to something scheduled or habitual in the past, but it does not so clearly mean was going to
• iba a despegar is closer to the English idea in this sentence

So here, El avión iba a despegar a las siete means the plane was scheduled/expected to take off at seven.

This is a very common Spanish pattern:

• imperfect for background, plans, expectations, or ongoing past situations
• preterite for the completed event that actually happened

Here:

• iba a despegar sets up the expectation: the plane was going to take off at seven
• despegó tells you the real completed action: it took off

So the contrast is:

• expected plan: iba a despegar
• actual completed result: despegó

That is exactly why the two different past tenses work well together in this sentence.

Because the structure is:

• ir a + infinitive

This is a fixed grammatical pattern used to talk about the near future or, in the past, something that was going to happen.

Examples:

• Voy a salir = I’m going to leave
• Íbamos a comer = We were going to eat
• El avión iba a despegar = The plane was going to take off

So that a is required as part of the construction.

In Spanish, when telling the time at which something happens, you normally use:

• a la for one o’clock
• a las for all other hours

So:

• a la una
• a las dos
• a las siete

That is why the sentence says a las siete = at seven o’clock.

Al is the contraction of:

• a + el = al

So:

• al final literally looks like at the end
• idiomatically, it usually means in the end, after all, or eventually

In this sentence, pero al final means something like:

• but in the end
• but eventually

It introduces the final outcome, often after expectations changed.

Sometimes yes, but they are not always identical.

• al final often means in the end / after all / when everything was said and done
• finalmente often means finally / eventually

In many contexts both are possible, but al final often sounds more natural when contrasting the original plan with the actual outcome, as in this sentence:

• Iba a despegar a las siete, pero al final...

That gives the sense of that was the plan, but in the end...

Despegó is the pretérito indefinido form of despegar for él/ella/usted.

• despegar = to take off
• despegó = he/she/it took off

Since el avión is singular, Spanish uses the él/ella/usted form:

• El avión despegó

The preterite is used because the takeoff is seen as one completed event in the past.

In this sentence, despegar means to take off for a plane.

More literally, the verb can also mean to unstick or to detach from a surface. The basic idea is something separating from something else.

So for an airplane, despegar is the natural verb for to take off.

Con retraso is a very common Spanish expression for transport being delayed.

• con retraso = delayed / late
• con un poco de retraso = with a bit of delay / a little late

This sounds especially natural for planes, trains, buses, etc.

You could say despegó un poco tarde, and people would understand it, but con un poco de retraso is more standard and more idiomatic in this kind of travel context.

Because un poco de means a little bit of.

So:

• un poco de agua = a little water
• un poco de tiempo = a little time
• un poco de retraso = a little delay

The de links un poco with the noun that follows.

Yes. That is perfectly grammatical.

The original sentence adds two small details:

• al final = in the end
• un poco = a little

So the full sentence gives a slightly richer meaning:

• it emphasizes the final outcome
• it says the delay was small

Without those parts, the sentence is still correct, just a bit less specific.

In this sentence, El avión is natural and useful because it introduces the subject clearly.

Spanish often drops subject pronouns like él or ella, but nouns are not usually dropped unless the context already makes them obvious.

So:

• El avión iba a despegar... = natural when introducing the topic
• Later, if context is clear, Spanish could omit the subject noun

For example:

• El avión iba a despegar a las siete, pero al final despegó con un poco de retraso.

The second clause does not repeat el avión, because it is already understood.