Each trip card shows small type icons for what's on the trip. Their color reflects your Trip Planning checklist for that trip:

• Green — this is on your checklist and it's booked.
• Red — this is on your checklist but you haven't booked it yet.
• Neutral (gray) — it's on the trip but wasn't something you flagged to track.

So a red plane means "you still need to book a flight for this trip."